Do NPCs just stay where you leave them or do they go back to where you originally found them?

It depends on who they are - but mostly they stay where you leave them

I've found all the NPC's I've released stay where I have dropped them unless they don't want to come back into the group.

It depends on your Reputation at the moment you drop them: If they're happy with the way you've been running the party, they'll stay right there and wait for you to come back. If they've been pissing & moaning about how you've been doing things, they'll call you names and walk off, and you'll never see them--or anything they're carrying--again. This, at least, is the general rule.

Oh crap! I left Imoen in the middle of the High something forest when I got that ranger dude. Oops. My character is a theif so I don't need her, but I didn't want her to be killed (role playing aspect).

Also, ya know the theif with the boots that wants to keep pickpocketing you. I got his boots for him, got the gold and exp. Talked to him again and he still pickpocketed me! So I killed him and took the boots. It didn't lower my rating and none of the NPCs complained, so I guess it was OK to kill him.

I somehow found the boots of stealth twice in the same game- without cheating or exporting and importing - i think both times was South of Beregost

Don't worry about leaving NPCs out in the cold, as it were: As long as they're not currently members of your party, monsters won't go hostile to them. Of course, that still doesn't make it a very nice thing to do, though.....

There are actually 2 pairs of Boots of Stealth in the neighborhood of Beregost. If you've been roaming around from map to map and doing subquests, you probably just found the 2nd pair. If you're positive you found both of them on the same map, however, you have a bug that gave you an extra pair of Boots. Lucky you--if you have a party member that can use them.