Stuck trunk lock solution W123 with factory alarm
 

If your car has a factory alarm system and all of the sudden you can't open a previously operational lock I have a solution.
This happened to me yesterday. I tried to open the trunk but I could not push the button. The lock still worked and the vacuum actuator still worked. I realized that it was something related to the alarm system.
Under the rear seat cushion on the driver's side at the front edge of the seat ledge there is a small diameter cable with a loop. You grasp the cable sheath and while pulling the loop at the same time have a helper pull up on the trunk lid.
Now that the lid is open, pull off the cover that accesses the vacuum element. You will notice at the top of the opening the end of the cable that you pulled on. Attached to the end of that cable is a motorized actuator that restricts the button from being pushed in. Pull back on the actuator toward the driver's side tail lamp and that will allow you to now push the button again. At this time pull the connector off the actuator so it doesn't lock you out again until you decide whether you want to fix this thing.

what are some indicators of a previously installed alarm?

Run a vin check on the Russian site and that will decode what options you might have.
http://old.mbclub.ru/mb/notes/parts/?lng=eng&id=9&idpart=4
The only visible indicators are an extra pin switch under the hood at the firewall and an extra horn just below the battery tray.