Should be your ambiant temperature monitor. It goes off at 2.5-3 degrees, can't remember exactly how much. The point of that is to warn you of ice on the road if it's wet.
But if you say you heard it within the past few months the tempertaure should not have dropped to 3 degrees.
The only other thing is you set the computer to tell you 1 minute before the next hour to beep. You can also turn this off.
If it isn't any of these, don't know what else it could be.
'02 C32 AMG / '07 C280 / '07 CLS550 / '15 JKU Rubicon