A while back, I was scrolling on Amazon in my bunker (using my bike generator to keep my laptop going) when I saw the BEST gadget: the Cyalume ChemLight Military Grade Surface Trip Flare. Basically, you set up a trip wire, and when something (human, zombie, etc.) sets it off, a reinforced glow stick is lit up.
Thankfully, Amazon still made shipments for a while when the apocalypse first began and my Prime hadn’t ended before I bought this device. It has done wonders, as you can read in the review I posted: