Be polite, truthful (without over-sharing) and record the meeting.
Often times, I forget to turn off my digital voice recorder after lectures. I find recording and typing notes later to be easier than frantically jotting down notes; also even I can't read my own handwriting. If someone were to bump into me and have a chat, that's on the recording as well.
Some time ago, I got a visit from two gentlemen from the FBI. They were very polite and respectful, even though my mom was very scared, they reassured her that this was just an exploratory meeting. They also prefixed my name with "Mr." which was a bit of a delight for me because I was still a teenager back then and used to "hey you".
I think I probably still have their cards somewhere.
Nothing really happened. They got an anonymous phone call regarding something I may have said in school and wanted to see if there was something to it. Turns out it was a panicked busybody (and this was after 9/11) and they weren't even that clear on what was said. It wasn't anything even remotely requiring a lawyer's presence.