The Thugs (thuggee was what they did; I covered the basics of their history, activities and historiography, including the ongoing historical debate concerning their actual existence, in an earlier post, here) were "discovered" by the British on several different occasions, and the level of awareness of their activities veered between almost total ignorance in the decade 1800-1810 to what might almost be termed over-familiarity by the end of the 1830s. I mean "over-familiarity" in the sense that, by that point, it's reasonable to suspect that a moral panic had established itself in India, and that some more common-or-garden bandits, and even completely innocent people, were being arrested, identified, tried and punished as Thugs.

But it's also important to bear in mind that – very much unlike the fiction depicted by John Masters in his novel The Deceivers, and in the movie based on it – there was an almost 100% disconnect between British colonialists and the Indian criminals they hunted and hanged. The historical record (which I am pretty familiar with, having read my way through a substantial portion of the estimated 60,000 pages of manuscript materials on the Thugs in the East India Company collections) is definite on this point. There is absolutely no evidence of collusion and collaboration between colonial officers and Thugs in the sense that your question implies – that is, of British officers conspiring with Thugs to prevent instances of Thuggee from being exposed, and of allowing known Thugs to continue their activities unmolested, whether in return for bribes or for any other reason.

Instead, the Thugs' ability to keep their activities secret depended on two things. First, they were highly mobile; Thug gangs were geographically, more than socially, exclusive, meaning that their existence and activities were fairly common knowledge in the remote villages that a significant proportion of them lived in, but remained unknown to the mass of Indians who lived in the areas they worked in, because these were in some cases 200 to 400 miles away from home. Second, the modus operandi that distinguished the Thugs from other sorts of bandits active on the roads of India was that they travelled in groups large enough to significantly outnumber their victims, and that they invariably murdered every member of every group they robbed before they robbed them – leaving essentially no witnesses to their activities.

With regard to British knowledge of Thuggee: there were a couple of early encounters with the practice. The unidentified bodies of victims of highway murder were found in some numbers from time to time – 28 were found in the Doab in 1808, and 39 in the same district in 1809, not all of which were necessarily victims of the Thugs. The earliest significant mentions of their activities occur in court records from Etawah, a town in the Doab, and date to the discovery of bodies dumped in wells 1810, and these incidents eventually led to the recruitment of the earliest informers who offered details of Thug practices and gangs:

Thugs worked the lands on both sides of the Jumna, from Lucknow to Jypore. There were, [Etawah magistrate Thomas] Perry’s informants thought, about 1,500 of them living north of the river, and more to the south, ranging as far east as Benares – far more, certainly, than the magistrate had ever imagined existed. They lived under the protection of the zamindars [local landowners] of their home villages and worked in small groups, leaving their homes ‘whenever they have nothing left to live on’. The more experienced members of the gangs killed by ‘strangling with any part of their clothes’. Less expert Thugs sometimes poisoned their victims with datura – the finely ground seeds of the thorn apple, a deadly relative of belladonna, ‘which deprives the object of his senses, when they plunder him’.

Several of Hossyn’s fellow Thugs, arrested along with the boy, eventually confessed to committing numerous murders over lengthy criminal careers. One, a 60-year-old man named Dullal, had strangled 15 or 16 victims and taken a considerable quantity of loot: ‘Much have I plundered and expended, beyond all account.’ A second Thug claimed to have witnessed 50 murders in only three years. A third had killed 45 men in eight years, strangling all of his victims with his bare hands.

These men’s gang had been led by a man named Ujba, who lived a few miles outside Etawah and supplemented his income as an armourer with the proceeds of Thug expeditions into the Doab. Gholam Hossyn had joined this group after spending a short period as an agricultural labourer, perhaps because he found the prospect of life on the roads preferable to that endured by a peasant farmer. His most recent expedition, at the beginning of the cold season of 1809–10, had opened successfully; Ujba and his 15 men lured their first victim to a nullah – a dried-up riverbed – a short distance from the road, "and murdered him in the following manner: Ramsooth, inhabitant of Dultua, strangled him with a handkerchief; when he was senseless one of the party inflicted wounds with a knife in both eyes and another wounded him, in the same manner, in his belly so that no person might recognize the body. They then buried the corpse in a nullah about a mile to the left of the road."

The dead man was carrying 100 Benares rupees, two turbans and some other clothes, which Ujba’s men appropriated. The gang then left the area, going on by forced marches to evade any possible pursuit, until they met two Afghans, whom they befriended and persuaded to accompany them on the road. Having rested for the night, the gang and their intended victims arose very early the next morning, breaking camp four hours before dawn at a time when they could be sure there would be few if any other travellers on the road. The little party walked a further six miles, halting for a rest at a spot well away from the nearest settlement. The Afghans were then cut down with swords. The first was run through while he was relieving himself; his friend was hacked to the ground as he attempted to flee, then finished off with repeated stabs to his back and neck.

One thing that emerges fairly clearly from this early account is how little resemblance some Thug murders actually bore to the modern conception of their crimes, which is that they killed stealthily and without shedding blood, and the truth seems to be that – outside that key MO of killing first, and robbing only afterwards – there was not always a great deal that different groups of Thugs had in common; certainly the word denotes what amounts to an occupation, a sub-category of bandit, in fact, and not to membership of some India-wide secret society of murderers or of a religious cult dedicated to killing in the name of Kali, the Hindu goddess of destruction.

In part because of all this, it took a further two decades – together with the publication of two articles in the journal Asiatik Researches and several additional encounters between British magistrates and Thug gangs – for the idea that such groups existed to become fairly generally known to the colonial authorities, or accepted by them.

The man most responsible for publicising the idea that Thugs did exist as distinct, definable groups, and ought to be pursued and prosecuted, was a young middle-ranking officer called William Sleeman, who encountered Thuggee while stationed in and around the central Indian town of Jabalpur in 1829-1830. Sleeman developed a system for recruiting informers from groups of suspected Thugs, and a paper database of information about the gangs, their members, activities, and also genealogies, that he used to track them down and arrest large numbers – several thousand men, in total – over a Thug-hunting career that made him famous, won him promotion, and occupied him for more than a decade.