[microbet]

You have most of the possibilities, but unless you know the villian you can't rule anything out. Some people enter a pot with the intention of bluffing it. Even with the intention of doing a multi-street bluff. Some of these players have gone on to do things like do really well at WPT tournaments. Some people overvalue some hands that you didn't include and then think you are bluffing because either the board paired or there is a flush draw or whatever. But, basically, just the good possibility of a lower flush here or a bluff after missing, is enough to make the call.

I think you could find a fold against a very predictable 2p2er whose limp UTG almost by itself guarantees mid (maybe small) PP. For some players that limp guarantees it and for some the only other possible hands would be AQs or AJs and with the Ace out in the flush, that would rule them out from having a flush. I don't think that player plays a middle pocket pair that doesn't fill up on the this hand in this fashion.