Still a tadpole myself (30 caches - all urban easy finds), but I have alread found 2 homeless homes and a crack stash. The drugs were in a zip lock about where I expected to find the geo-cache. Thought it might have been muggled, but then looked in the baggie.
As to the homeless, just have to treat them like other muggles, don't let them see you retrieve or stash a cache - they will probably muggle it to see if there is anything there they can pawn.
As for the drug stash, walked out of the park, made a call to the police from a pay phone. I didn't really want to explain how I found a stash of drugs in a drain pipe under a park pedestrian bridge.