Yes, with a caveat. My car is old enough that it has a small stopcock for the block drain. So a quick twist of the valve, and the block drain is open. But the actual opening is relatively small (compared to what I imagine most of you have) so it is not like water gushes out of this hole. It just flows out as a small stream after you open it. It takes a long time for the system to empty through that little hole. If I come back to the car 30 minutes later, it will still be flowing, but at a trickle compared to when it started.

The FSM says my car has a coolant capacity of 11 liters. I can add about 10 liters of water before it is full, so there is still a liter hiding somewhere in there.

__________________
1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).