I read a lot of non-fiction books and they often references sources that are old notes between friends, enemies, etc. Sometimes notes refer to very detailed things, like the way a rock cliff looked in the sunlight or the way a tree branch broke.
These run counter to what I see today. Do people today still takes note to a level where future historians can piece together events? I know social media is a treasure trove of information but not a lot of it is detailed in the way old diaries were detailed.
Instead of looking through attics and libraries (how do researchers know which library to go to?) will they be searching endless databases?
Since there are two questions in the title, I will address each separately. Regarding the first part of your question regarding where historians find sources for little known events, this depends greatly on what period and topic you are looking at, the resources available as well as the amount of detail the historian hopes to find. What do we mean by "little known"? If it is the kind of exceptional detail such as how a rock cliff looks in the sunlight, it is rare for modern day historians to pay attention and to write about these small details as we try to be concise. Often, the weather would not be crucial to the argument. This may also have been the case for contemporary writers who may have considered those details unimportant.
Nevertheless, if one wished to find small details such as the weather that day, it is not impossible. The kinds of sources that come to mind in terms of containing extreme detail are letters, diaries as you mentioned already, elaborate literary works describing a great event at the time, or even newspapers if the period you are investigating comes after the sixteenth century. The old notes between friends or enemies which you mentioned are interesting and sometimes may not prove too difficult to be found depending on the importance of the people in question. Bureaucrats for example were highly likely to store these personal letters safely and future generations would have treated these with greater care than they would a common persons. Robert Hart, an Irishman who worked for the Chinese Customs in the 19th century, for example kept large quantities of personal letters to his colleague in London (see two volumes of The I. G. in Peking: Letters of Robert Hart, Chinese Maritime Customs, 1868-1907) and even a diary. These have all survived and all of those links will link you to the available items. Robert Hart had a particular penchant for gossip which is reflected in both these types of materials, which may also be considered as "little known events".
However, if the "little known event" you were interested in is an event that simply is rarely known by the common layman due to the fact that it is rarely discussed in modern day society, records may be far easier to find and far more rich. For example, if one wanted to know about the story behind the founding of the now globally important bank, the HSBC, in 1865, one could read the HSBC archives and newspaper articles. This was precisely what one author, Frank King, did in writing three whole volumes on the little-known history of this bank.
How does a historian know where to find these? Footnotes and communication with other historians in combination with guesswork as to where any additional extra detail may lie is an important part of researching history. Furthermore, in our modern day world, we are very fortunate to have digitized databases which have search engines which makes our jobs a lot easier.
About your second question on whether people these days still take detailed note of little known events, this is a question which cannot be answered without an entire study and more precise definition of what we mean by "little known events". If we mean details such as the rock cliff or a tree branch, it would prove impossible to physically count how much detail every living person wrote. What we do, however, know is that people of a highly literary character doubtless continue to exist today and record the news or events in their own lives in great detail. Sensationalised news these days contain all the detail that can be acquired including photographs (see tabloid magazines particularly).
(To the moderators: as this is my first post on reddit please inform me if I have disobeyed any of the many rules of answering questions)
Is this a decent guide on how to read source material for retention purposes?