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Guns. 
Probably a cat. But since I’m at work right now, it’s impossible for me to know for sure.
Several poweradaptors for charging our electronic stuff. Nothing else, I don’t want to provide a safe haven for monsters or dustbunnies.
A rifle, in a case and unloaded. A blue exercise mat. Dog and cat hairbunnies. Probably a sock or two or possibly a stray t-shirt. An extension cord that my reading lamps are plugged in to.
Dust bunnies and a dog treat.
Hey, me too!
That, and a spare laserdisc player and a plastic lidded tote filled with less-often-worn clothes.
Nothing. Not even dust bunnies. I know cuz I vacuumed under there on Monday.
Rocket parts, a sextant, and dust bunnies.
Enough fur to make another dog entirely, and summer clothes in shrink wrap.
Dust, and the leaf and pads for my dining table. A rolled up wool Oriental rug. A jewelry box containing my college ids (I’m 55) and a Who (the musical group) pin. A kit to make mini sock monkeys. A round mirror and 4 placemats in a box. Probably a few candy bar wrappers.
About 800 Betty Neels romances, I think (not mine). Enough paperback romances so that the legs of the bed cometimes come up off the floor.
So your cat is both under the bed and not under the bed until it is observed.
Slippers.
But by the time the cat is observed, we will not be able to know how long previously did his trajectory pass under the bed.
Under the bed at the hotel: a carpet. Hopefully, nothing else, although seeing how “delicate” the cleaning staff is, who knows.
Under the bed at the shared flat where I spend weekends: dust bunnies, probably; I really need to mop up.
Under the daybed I use at home: a foldable bed. Probably no dust bunnies, I’ve never found any when taking the foldable out.
A bunch of clothes and random junk that got put in to under-the-bed storage bins about 10+ years ago. I moved them to my new house 7 years ago and put them right back under my bed. I actually open them once every 2-3 years but never use or throw away any of the stuff.
And dust.
Boxes full of photos, memorabilia and assorted ephemera (aka junk normally found in an attic)
Dust, possibly a sock or two, and a lockbox containing my important personal documents.
A trundle mattress.
A 16 gauge pump, loaded and ready. Probably a sock as well.
Nothing - we have a pedestal bed.
In the drawers that make up the bed base there are several sets of sheets and pillow cases, some spare blankets, the electric blanket, and my SO’s martial arts belts.