I think Jeremy's "anti-hoarding" rule is an excellent suggestion to keep the game moving.
Also, in the event of a double-move, perhaps it would be a good idea to prevent the same player making both moves? Otherwise I think there would be a lot of scope for cheating (i.e. player puts bug in cache, hides the cache in a more "difficult to find" way than it's normally hidden, and then comes straight back after the time limit and moves the bug again...)
Also, it would be fairest if the 6 hour/midnight limit starts from the moment that the player LOGS the movement, not from the actual time that the bug was placed.