I think you found your problem.
Btw, I DID have a battery venting problem on my W124 years ago, and the smell was sulphuric acid vapor...VERY toxic AND dangerous, especially the propensity for the battery to explode at some point!!!
Turned out, the voltage regulator had failed, allowing the alternator to overcharge the battery. I put a test lead on it later and found it to be 17.5 volts!!!
