Welp. Here I am, playing with these various 18650s, thanks to this new ... er, hobby.
Testing them in a good light (Convoy S21E), had it standing on its tailcap.
Noticed it's actually a pretty good way to see if there vapor left on something that's about beat. Thin vapor is very hard to see, to me, though it medicates, I guess. Exhale in the direction of the beam and it lets you know.
Technique on these things has to be learned; I was plagued with thin vapor in one machine, I about gave up. Anyways, try a flashlight.
CR