It happend to my SA F10D as well when once water went inside it (while car parked, everything off) and i took da amp off n opened its cover n kept it in my room for a week to dry, but when conncectd back, it was draining my brand new battery even wid da system off.Like a fully charged battery in 1 night parking.
My car electrician guessed tht too cuz with engine n everything off, battery terminal disconnectd n connectd bak was giving a heavy spark (not normal). It didnt happen when amp +ve wire was taken off, spark was little.
So its either a bad moist amp or wrong wiring or old battery...If wiring is fine and its the amp, use a switch in +ve wire but a nice heavy duty one.