This is coming from a noob but i figured out you situation, im in the same boat.
When you pull your hand out of the tank, a few drops will drip off and slide down the glass. you dont even notice it, it settles at the bottom right on the rim of the bottom trim piece, you dont see it cuz its wet and clear, a few hours later you see the salt after the water has evaporated.
Im also running a penguin bio-wheel type filter up top in the back, and even though i have the top nicely covered all the tiny little splashes that i cant even see, deposit a little bit of salt water at a time, this water keeps evaporating but leavs a constantly building amount of salt...
I tried draping a towel over the outside of the tank last time i added some mushrooms and feather duster, and this time there was no salt on the bottom.
Im almost sure thats what you have, dont worry about a leak, if there was a leak you would see salt on your stand and elsewhere...
i hope that wasnt a run on lecture.. hope it helped.