I've seen a bunch of bike packing videos with people holding a bike tube up to their ear or using water, trying to find location of a leak. In Yvette Kooijman's latest video her partner used something entirely unexpected.
https://youtu.be/A-v_hl53x9I?t=503
Just a small container with styrofoam beads above wire mesh and a clear plastic top.
https://hollandbikeshop.com/en-gb/bicycle-tires-and-inner-tubes/bicycle-tire-repair/puncture-repair-kits/simson-puncture-finder/
