Lightning is not a local thing only, remember the little Franklin sensor hears about 40 miles around you and a heavy ‘spark’ 10 miles away could trigger it. Winter and summer are nice periods where so many heathers/coolers start running way more than in spring or autumn … those are often nice candidates to trigger false positives. Maybe a neighbour bought a new device (cheap thing mostly) that is far from ‘clean’ and triggers (leds, fluo lamps, a pump …). Not so easy to filter all out without becoming completely deaf … if you have a little am radio, tune it to a free frequency and walk around the house already and listen for potential disturbers (transfo, led etc)… maybe you can already find a local source