Coolant leak (only) when engine turned off
Recently I found the coolant in my 2011 Focus mk3 seriously low. I also found traces of dried coolant in the vicinity of the reservoir tank. I started following what's going on more closely and found what to me looks like an odd behavior:
I top the reservoir tank to the max mark. Drive - quite a lot, in warm weather. Stop car, keep engine running, open bonnet: reservoir seems to not have lost a drop. Now, shut down engine. Within like 5-10 seconds, pressure builds up and fluid starts leaking; at least part of it seems to be from the release cap, not sure it's the only place. In a short while, there's a big puddle of coolant on the floor under the car.
Car cools down, process repeats. Fill fluid, drive (slow traffic, should be stressful enough for the cooling system). Nothing leaks. Park car, stop engine - psssffffssssffff. Puddle, lots of fluid pours out.
All of that time, temp gauge is rock solid at a hair below half.
No water in oil. No apparent oil in coolant.
Anyone seen this? What's going on?
Thanks for any insight.