Ok, so, Lucan and groves. u/jimthewanderer has already provided some good stuff about Druid stone circles, and I'm not an expert in that field, so won't delve into it. However, I do have something to say on the subject of Lucan and groves.

First of all, as you know, Lucan is not really a historian, he's a poet writing historical fiction.

There is a famous grove in his poem, De bello civili, outside of Massilia (Marseilles). The account of its destruction at the hands of Caesar, from 3.399-452 is one of my favourite in all of Latin literature. The grove, or lucus, is characterised as being unviolated by bronze, a key feature of Roman ancient woodland (see Hunt, Reviving Roman Religion, 125). This grove (lucus, not nemus) is located outside the city, in a large area of woodland. When Caesar arrives, the woodland rapidly decreases as he fells it for siege weapons and camp building. Massilia was under siege, but their sacred grove remained.

This grove has a beautiful description in Lucan, who is writing at his very best: it is dripping with gore, the home of human sacrifice, and only the priests dare enter. Caesar's own men, although not part of the religion, refuse to take an axe to it, and it eventually falls upon Caesar himself to fell the first tree (Phillips, 1968 explores the problems of killing a sacred tree, as does Augoustakis, 2006). He drives his axe into the violated wood, and Lucan is deliberately ambiguous about when the violation happened. It's either at the point of the axe, or of the human sacrifice.

Anyway. In answer to where groves were, we have to first ask: what makes a grove sacred in the Roman/Druid world? The answer is that we don't know. Because the Romans didn't know. See the below, from Ovid's Fasti (4.749-55):

‘If I have grazed my flocks in a sacred place, or rested under a sacred tree,

And my flocks have taken food from graves in ignorance;

If I have entered a forbidden grove (nemus… vetitum),

Or the nymphs and half-goat god were put to flight from my eyes;

If my pruning knife has ransacked a grove (lucus) for a shady bough,

From which a basket was filled with leaves for a sick sheep,

Forgive my fault.’

So where were these groves? First, we have to ask what is a grove? Yes, some of them had some markers, like those mentioned by u/jimthewanderer, but a great many didn't. Again, Hunt goes into some detail about the unknown sacred tree in her monograph.

Largely, we don't even know what makes a grove, or lucus. Broise and Scheid (1993) argue that it stems from lux, meaning 'light' but Dumézil (1975) points out that the word is used for light places and wooded ones. As for nemus, this is more commonly used for an organised grove/sanctuary, of the sort that u/jimthewanderer has already told you about. But it is not the more common descriptor in Latin, where your question began. Lucus may refer to your idea of a 'wild' grove, although it can be used interchangeably with nemus, and refer to an organised one, such as that of the Arval brethren, outside of Rome.

In their inscriptions, they tell us that they took iron into the grove to coinquere the trees. Hunt argues that this word, which is only found in these inscriptions, refers to a ritual pruning to maintain the light space that she argues defined the grove. I argued differently in my doctoral thesis. There, I argued that coinquere actually refers to a ritual tying back of the trees' branches, in line with the religious requirement that the trees remain uncut, while creating the light space, but also allowing the grove to be a shaded woodland.

An advantage of this is that a grove can be created anywhere, a disadvantage is that we will probably never know every location of every grove.

After all of this, and if you're still reading, I would like to point out that the only non-Roman grove I have mentioned is the one outside of Massilia. Groves, and trees in particular, are very common in Roman ideas, both religious and not. They were not restricted to this weird idea of paganism as they are today. The sacred tree was an established part of Roman identity, and by no means something that was frowned upon by Romans prior to the writings of the apologists.

You've got a few questions here, so I'll tackle them each individually, and then tie it all up at the end.

I think it's worth mentioning that Romans where absolutely awful at describing other cultures, and classical accounts of Gauls, Britons, etcetera are full of nonsense (A fun example is the suggestion that the natives of Scotland lived in Bogs with only their heads above water). As such it's always worth approaching anything they have to say with a nice pot of salt.

In particular, Lucans account is poetry. He was a poet, not a historian. As such his description is almost certainly pure fiction apart from the general concept of a sacred place outdoors.

Where are these groves?

Firstly, the groves, or Nemeton, are simply sacred natural spaces. Perhaps enclosed by earthworks, or postholes indicating fences. Some have been found with altar stones, and Romano-British Examples even have inscriptions in Latin. Many place names in Europe also derive from the word Nemeton, indicating the presence of sacred places in antiquity. The groves and temples of Iron Age peoples can be found across Europe archaeologically. However completely "wild" groves don't survive archaeologically, if they ever existed.

did they move around?

The Druids probably moved around, but sacred spaces are typically sacred spaces for a reason. Whether a sacred place in a woodland would be replaced or renewed if the trees where damaged in a storm, or succumbed to disease is simply not known. New sacred places would also be established to service new settlements.

The idea that the Celtic peoples had no temples is also a questionable claim in the first place, there are structures found archaeologically that resemble temples and shrines from the pre-Roman Iron Age.

Is it possible that the Druids existed a lot longer than we know

Yes, it is certainly possible that an such an esoteric and poorly understood religious order could have existed earlier than we have written evidence for. However that is purely supposition. We know barely anything about the Druids, so we must refrain from making any such assumptions.

could it be possible that stone circles could mark the location of these groves?

Almost certainly not.

Stone circles are almost all associated with the Neolithic. Neolithic cultures across Europe where seemingly replaced by the so-called "beaker people" culture with the advent of the bronze age. The "celtic" cultures emerged in the later bronze age and Iron Age, and the only evidence we have of the druids comes from Roman written accounts.

We do know that Stone circles where of interest to later cultures, but the idea that this was a contiguous tradition focusing on their use as sacred spaces is completely unsupported.

One thing you might find of interest however, is that many stone circles from the neolithic where preceded by timber circles. And indeed, some timber circles are contemporary to stone circles.