I had a similar problem, I thought it was evaporation and topped off the top inch for three days strait. Salinity dropped and all my inverts died, the ammonia spike caused all my fish to get sick and most of them died. Now I started a new tank and made sure that wont happen again.
I suggest trying a tank switch A.S.A.P, or keeping your fish/inverts at the LFS
until you can plug it.
Salt will build up around the hole, look for that and you'll find the leak, if it is at the bottom you may not find it, and probably can't plug it.
P.S check your salinity every day!!!