Had the same problem once, and I don't have an alarm! I'm guessing that the alarm setup is basically in the Benz DNA from the factory (sorry I'm not being technical here) and is activated if something is amiss with the locks. At the time, the key cylinders in both of the front doors were broken and I could only lock through the trunk; I'm guessing that, when I took it a shop, someone locked/unlocked through an open window and didn't lock through the trunk (which was the only way I cold lock the car at that point).
After seeing the lights go through the blinking routine a couple of times while driving home from the shop and then doing another startup in the garage, I shut the car off and did the proper lock/unlock from the trunk a couple of times. That seemed to reset it.
If all else fails, disconnect the battery, wait a minute or so, and then reconnect.