My bet will be the power supply as well. i had one HC-SR501 running from battery for a few months .False triggers? every time sun comes out from a cloud! for outdoor they are crap.
Just for you know about power supplies , last project that i build a k type thermometer(for high temperatures) on my fireplace, i feed it with a HLK-PM01 and while I didn't connect negative terminal to the fireplace steel chassis(ground) , it fluctuate a lot during readings, like 20c then 45c,25 etc.. not even with a capacitor.
