Don't know how the weather has been in sunny California but if you've been in a rainstorm that could be the source. The other is if no rain and lots of sun....and the use of your A/C, it could be a plugged evaporator drain line. Is the fluid clear and not sticky?
Another source of water, contaminated, is your heater core... a wild shot, but anything is possible. Ck. your coolant level.

Tobias MB
300CE, C280Sport, 190/5.0