A couple of years ago, I was at a conference and there was the typical Q&A after someone gave a talk. I can't even remember what is was about, however in the audience was one of the big-shot researchers in my field (papers, citations, grantmoney). She made a comment, that kept stuck in my mind, and now I would like to borrow that information for my thesis. Can I cite this as a personal communication?
Note, I have not talked personally to her, just was in the audience and "overheard" the comment. Note also, what she said was related to some government research contract, and is not published.
Edit: some clarifications:
It was a public comment on the talk, made to the whole audience.
She mentioned that they are using a certain instrument in their research (the government thing), which I like to mention in order to point out the relevance of that instrument even in other areas than what my own reserach is covering.
The problem with citing an off-the-cuff comment of this type is that there is no way to know whether you are even remembering the statement correctly, let alone whether the statement is truly reliable.
I recently had the experience of discovering that a "factual" belief that I had held for a long time was actually based on an incorrect interpretation of the trusted source that I had learned the "fact" from. Nobody was intending to deceive, but they had misremembered their own original source, and therefore passed on subtly incorrect information. Other times, a person may simply misspeak, particularly in a conference situation: for example, I said an incorrect fact during a Q&A of a large talk last year, and did not realize that I had said the wrong thing until later, when somebody asked me a question about what I had said.
I thus strongly advise against citing as "personal communication" anything that wasn't the result of a careful discussion. Instead, you can get in touch with the original source (or somebody else likely to have the information) and ask them if there is a better citation you can use, or even just your memory as the inspiration for some Google Scholar sleuthing of your own. After all, if people were using the instrument in area X a few years ago and it was useful, there will almost certainly be some sort of documentation of the fact by now, even if only in technical reports rather than published journals---and if not, you have to wonder how useful it really was.