I thought you were talking about geocaching for a second...

In any case, make sure YOUR cache doesn't look like a geocache (which are sometimes in used ammo cans!) and seem to be hidden everywhere. You might inadvertantly put your safety cache in an area beside a geocache and be found accidentally. Just saying...

Secondly, locations such bridges, utilities, schools, etc. are also not good places lest someone find it and think it was a bomb and then call out the squad. That happens all the time in the geocaching world.