Replaced the ECM 2 times

I have replaced the ECM (on firewall) 2 times because it visibly (rotten egg smell)failed only after I let the vehicle sit for approx. A month. Any thoughts on why the ECM on my 1994 Dodge 5.2 B150 burnt out (capicator rotten egg smell). It ran fine I parked it for a week It cranked but no engine light ect.I could smell the Ecm opened and saw burnt capicator on circut board.

I replaced Ecm ran perfect and 3 months later after not using vehicle for a month the same exact failure happened again with the second ECM. FYI the failures occured when the vehicle was not being used. The Battery both times drained low. Any idea as to what circuts could be active or loading thru the ECM when the vehicle is off with the ignition key removed? Thanks for any response