I think you got your hands on one of the first wireless models: I think they had only one battery to start with. The battery inside the Powermeter will loose current with time - also when it is left unused for a long time.
Regarding mosture - I had a wired SRM some years ago that worked fine - but when opening it for a batterychange it actually had some water inside. The guy that helped med with the batterychange had it open for a day and made sure it was thoroughly dried before making a batterychange, and sealing the unit again. It worked fine afterwards... He told me it was a good idea to give the seal some silicone-grease to make sure the powermeter stays watertight... Of course water is not good for the unit: But SRM's are quite wear resistant in my opinion.
On the other hand - SRM Germany once "forgot" to install the seal when they serviced an SRM for me years ago... One ride in the rain and and I could actually HEAR the water splashing around inside the cranks. THAT killed the powermeter
Fortunately SRM gave me a new powermeter so everything went well after all
Dura ace SRM's have ONLY been made with 8 strain gauges: so no need to worry (or calling the FBI