W201 door lock oddness: locks release over time, still has vacuum

[gastropodus]

My W201 has developed an unusual door lock anomaly: I always lock the car when I leave it on the street in front of my house. After several days, I'll notice that the locks are all up and the car is completely unlocked. Here's the weird part: if I take the key and turn it in the driver door, all the locks will pop down, meaning that there is still plenty of vacuum in the system! This is, of course, without starting the car to replenish the vacuum.

This is very different from my W123, which has a slow slow leak in the vacuum system: on the W123 the vacuum will be gone after a couple days, and when you turn the key in the driver door nothing happens on the other doors. They only come up after I start the car (to the annoyance of my passengers).

Any ideas? It seems like this is a problem in one of the front door lock assemblies: it lets go by spring force or something, and all the others follow?

Kurt