Think that's true, bookworm.
Several times, I've seen another aircraft that's crossed my track (in VMC) relatively close, and given no indication whatever of having seen me (ie if I hadn't altered course, we might have collided).
I'm sure that I've also been the one who didn't see the other aircraft on a few such occasions.
What I have noticed very clearly is that some folks are better than others at spotting traffic. One colleague who flies quite often with me will spot other aircraft long before I do.