Maybe I just like in a Pollyanna world where everything is sunshine, rainbows and ponies frolicking in fields of candy canes with gum drop fences . . . but in reading the logs my first thought was not that someone was messing with the caches, but rather that an inexperienced (and young) cacher might not have read all the "rules" of the game and thought you were supposed to bring home the cache as if it was actual treasure . . . moreover I would not put a lot of stock in the fact that he didn't sign the log since a person who wouldn't know the rules of the game might not realize he/she is supposed to sign the physical log.
If someone wanted to really mess with a cache there is a lot more they could do than simply go on-line and write some logs about taking the cache home . . . leaving bodily fluids/deposits behind in the cache, destroying the cache, moving the cache, leaving profanity-laden logs . . . all these are far more likely to occur with someone bent on messing with a cache.
And if someone was intent only on "messing" with a cache with their on-line log I think they could come up with something a lot more provocative than what they wrote . . .
Again, maybe I'm just a Pollyanna, but I'm wondering if this was a newbie who brought the cache home and then was told or learned that the cache was supposed to stay in place and it was returned. . . .
"Courage is not the absence of fear, but the realization that there is something more important than fear."
"Death is only one of many ways to die."