7 stacks iron, 2 stacks steel bars, 2 stacks gold, one stack diamond, out of the chests in my little base on the mountain on the island across from newcity. I distinctly had a sign not too touch anything, and someone ignored it and built a bridge to my island as well.
Sorry man, that sucks! You've got to hide chests that have anything important in them though - put them beneath your floor, behind a wall, etc. Just put them somewhere hidden, where you have to remove a few blocks to find it, and you won't lose anything anymore!
It's just too easy for someone to walk by, take a look, and grab a few things without anyone else knowing the better.