Years ago, I "lost" my first HDS, a CR123 Executive GDP+. This place had their electrical, networking, and DVR (but no camera for this room) in their break room. I was working there, and accidentally left my light, right in plain sight, with beacon mode on. I went back the next day, and nobody had seen it...except the person that decided they wanted it, of course.
Years before that, I lost my Fenix L2T somewhere in a ceiling cat walk, or the cable nests below. It had long since lost all modes but turbo, and was tint-shifting, so I didn't lose any sleep over it.
Years before that, I lost a Fenix L1P, at college, somewhere. The campus used to be a giant garden, with a fair size pond (now they've grown out of that, to fit more and more buildings), with many small parking lots and cement walkways, liberally peppered with storm drains. So, I didn't make too big of an effort to look for it, after not finding it indoors anywhere.
I did the same as the top one again, but at my house. I was renting a small old house in the city, and set the light on top of the main panel box. I went inside for a drink and a snack, came back out, and there was no flashlight. I think it was my Illumination Supply 219B HDS. I sure do miss the joys of city living
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