No real answer for you other than you have a leak somewhere.

My wifes 300D, vacuum locks disconnected under the hood due to vacuum leak, will lock the passenger side door almost immediately after you open it. I haven't had a chance to track down why though.

__________________
'84 300CD Turbo 132k (Anthracite Grey) - WVO - My daily driver - Recently named coo-coo-coupe by my daughter.
'84 300D Turbo 240k (Anthracite Grey) - Garage Queen
'83 300D Turbo 220k (Orient Red) - WVO - Wifes daily driver

I'm not a certified mechanic, but I did stay at a HolidayInn Express last night.