Can someone tell if I was logged on their Facebook page?

[abraXus]

it wouldnt automatically sign you out just because they signed in

but yes, it's called an "active session" and they can tell when someone else is logged into their account, and they can end your active session

they wouldnt know it was you, they would just know that "someone else" was logged into their account

unless of course they just think about the other places they have logged into, and one of them was say... at your house