Self contained vehicle door warning strobe lights.

These are an odd little thing. You stick them on the end of your car or truck door and when opened they start strobing as a warning to other traffic.
At first I wondered how they would fit in between the side of a door and the frame, but it turns out that on most cars there is a reasonable gap.
The devices use double sided pads to attach in place and use a magnet as a sensing device for when the door is closed. Each unit contains a single easily replaced CR2032 lithium cell and quiescent current is very low.

The units are available in a wide variety of colours. Here's a generic eBay search link for them. Target price is around $3.
https://www.ebay.com/sch/i.html?_from=R40&_nkw=car+door+warning+magnetic+induction+led&_sacat=0&LH_BIN=1&_sop=15

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