I do kinda of deserve the shellacking Karma gave me about this, I guess. I got 4 trucks pretty much like this all with a very close problem. We pulled the batteries and cleaned as much of the solid buildup of corrosion out from under and around the battery compartment, as much as we could and after that, we noticed a 'chirp' but not from the motion alarm, ours came from a peizo buzzer on the distribution card. I noticed some crud had gotten between the pins on one of the cards, and we cleaned it with an old toothbrush and that one went away. the others went away after about a week of drying out inside the warm shop with the batteries disconnected (I did not get around to checking them or anything, it was just the next time I got time to look at them they had stopped). I spent about 10 hours looking into the circuits on the only 1 I got to "fix". I think it may have been from the low pressure water that was used to rinse out under the battery box.
I hope the OP got his fixed by now.