I have a PIR sensor connected to a RP2040 Pro Micro and a relay. This is housed in a plastic box outside and controls a strip of LED lights along my walkway. This has worked for over a year without issues. It now seems that the PIR sensor isn’t working correctly, but only outdoors. I brought the whole thing indoors for some testing today and it worked flawlessly. I set it up outside and it will work for anywhere from about 1 - 4 triggering events (someone walks within its field of view), and then it doesn’t ever trip the relay again until I unplug it and plug it in again.
Has my PIR sensor reached its EOL?
Powered PCBs tend to corrode rapidly in humid air and especially outdoors, where temperature changes lead to moisture condensation on the PCB. They need to be “conformally coated” (e.g. covered with nail polish) to prevent or delay that action.
I would say that one year is pretty decent for the linked PCB, and would even go so far as to guess that you are in a rather dry location.
Interesting. Maybe I’ll get another one and some nail polish and try that.
As for location, nope, I live in the northwest of WA State. No one ever would describe this as a dry location. 
Thanks!
Another approach to reduce condensation is to keep the PIR enclosure at temperature a few degrees higher than ambient. For example the box might contain a power supply or other electronics. The PIR circuitry itself probably would not consume enough power to be effective in that regard.
In severe cases, all the circuitry is embedded in potting compound, e.g. an impervious silicon gel.
I’d sure be curious to learn why: which particular characteristic was fulfilled indoors to resume operation? One year seems poor for a low power, non contact sensor.
My guess is simply drying out. That happened with an expensive commercial GPS unit that was left out in the rain, and would not turn on. I put it in a drawer and after some months indoors, it functioned normally.