Yeah this keeps happening to me too. Started noticing it a few months back, always about 30-60 seconds before anything interesting shows up on the recorder. Like the equipment knows something's coming before I do.
My working theory is its some kind of EM interference from whatever's trying to make contact, sort of a precursor bleed. The entity or whatever it is needs to pull energy from somewhere and that process disturbs local EM fields first. Makes more sense to me than just blaming dodgy batteries anyway.
Anyone tried running a separate EM meter alongside the recorder to see if the spikes correlate? I've been meaning to set that up properly but haven't got round to it. Would be interesting data if the timing lines up consistently across different locations and different people's gear.