Had the same problem recently with a 2011 Mercedes. Didn’t figure it out until eventually the Keyless Go module (which allows push to start and automatic door unlocking) died. This fixed the parasitic drain.
Apparently when these sorts of modules are dying they fail to turn off and drain the battery by constantly searching for the key fobs. Incidentally while the battery drain was going on the key fob batteries were dying extremely quickly as well, also suggesting a lot more communication than normal was occurring.
