Originally Posted by Aztec Kid
...or more commonly, rain coming in the forward entry or service door when the aircraft is on the ground.
Having had to clean the Cockpit Grid Collector Assembly or "basin" as you call it (AMM TASK 53-19-00-210-001). I would be very impressed if any moisture in there is indeed from rain entering through the forward entry/service door as you mention. Especially if the flightdeck door is closed.
PaNmAn