I'd like to add to this topic by stating that I've been in a situation where I unexpectedly found a TB and when I went to log it as "Found", it wasn't even logged as "Placed". This confused me but I logged it as "Found" anyways. Sometimes people forget to log it as "placed" (or forget where they placed it....especially if they had a long caching day) and it's not necessarily taken or ignored.
On the other hand...sometimes people adhere too closely to the "Finders, Keepers" mentality, feel that they were just rewarded for their efforts and your beloved TB's/geocoins are gone forever. I hate that.
And of course, you have the people that strictly geocoin/TB hunt. I hate that even more.
An unfortunate side effect to the great sport of geocaching.
