Do like the owner would: keep it. Trying to find the owner of the place.... "Hi sir I found 100$ while exploring your abandoned building" Owner will probably question you more about how and why you were there. Cash is not like valuable object for somebody ex.: finding a watch with the name of somebody engraved behind, that I would make research to find the owner.

It would depend if it was possible to identify who owned it, and under what circumstances I found it. Some time ago while working on the trams I found $120 with a post-it-note attached to it saying "rent money". I figured the person who lost it needed it a heck of a lot more than me. So I handed it in. Then a few years later while exploring a well abandoned building a friend found $50. No way to identify who owned it so we kept it and put some of it towards petrol money as we were both students at the time.

I would never actively go searching for money. Like say I found a homeless person's stuff I would not search it for money. That's just not on.

say instead of cash you found a diamond ring or a real nice rolex, would you take it. provided there was no id with it, i think i would.

I don't think that whether or not an explorer would keep any money they found is necessary and indication of their level of ethics. Judging from the responses I have read it seems that whether the money is returned or not is a matter of practicality (whether or not the identity of the owner is obvious). I doubt that even the most ethical person would go to great lengths to return money found in the depths of an abandoned building unless the name of the owner accompanied the money. Given the number of people that will enter an abandoned building I would not be inclined to assume that the owner of the property is the owner of the money.

It depends where I found the money. If it was in an active site with people around me then I would try to contact someone and give them the money. However, if I'm exploring an abandoned building and I find 100 dollars then I would have to take it. It would be a lot easier doing that then trying to contact the owner over a 100 dollars.

I would keep the cash unless it was clearly marked who it belonged to. I would then tell no one of what I found.
Seems to me, regardless of your place in life (property owner, junkie, explorer) if you leave money in a vacant and run down building, you're not too concerned about what happens to it.