Is it possible for 385,000 gallons of water, in an 15 x 25' chamber
with =no= history of dampness from the outside, to simply disappear?

No one seemed to address this one.
I got 5' of water in my basement when a creek dammed up during a power
outage. When they undammed the creek, the 5' drained away almost
immediately.
So, without knowing anything about your basement, it seems entirely
possible.