Stranger's remote has the ability to unlock my car

Its not possible to reprogram the keyless code on your key but it IS possible with a mb tool to have all the keys disabled except the key your using or if you really wanted you could get a new unused key from a locksmith or dealer and then disable all the other key spaces so you could be 100% sure there is no trickery with old keys.

That said, It is highly unlikely with the system these keys use that someone else has a working key unless you have recently lost one.

More likely you have some issue's with your own remote, the controlling module for locking systems or keyless entry, or the door lock actuator itself is failing and thinks its being held in the unlock position sometimes (makes most sense since all windows are open). I'd take it to a mb specialists or dealer and have it checked out first instead.