Had the same problem with my '87 300E. So far, my 400E hasn't presented the same issue.
My problem was the alarm sensor/relay/whatever you call it....on the driver's door lock had a fault. Every time i attempted to unlock the car from the driver's door the alarm would trigger. I was able to unlock the car from the passanger door and the trunk without the alarm sounding.
You can have the sensor in the driver's door replaced or you can pull it out and unplug it. Unless the '92 is vastly different, you will see a plastic cap on the inside of the door...about the same level as the door handle... If you pull the cap out, the alarm sensor will come with it and you will see the wiring harness. Unplug it and the alarm problem will go away...BUT..The alarm will no longer set from the driver's door..
Hope this helps...This, at least, was a quick fix on my '87 300E...Have fun !