The zebralights I posted are quite small and easily fit inside a pocket, but if you are truly looking for a light that disappears the. You could check out some of the single cell AAA lights.
For a good blend of price and features I like the Maratac AAA rev 5. It’s small enough to fit on my key chain and yet still comes with a clip for pocket carry or clipped to the brim of a baseball hat. It’s constant current, not PWM. You can get it in cool white or the nichia 219c warm white emitter which I prefer for much better color rendering. 3 modes, medium, moonlight, and high. I’m not sure the exact water resistance rating, but mine has seen the bottom of a pool without issues. The build quality is top notch, it’s efficient, and it comes in several different flavors.(copper, brass, titanium, and aluminum)
AAA Copper Nichia 219 Flashlight by Maratac REV 5
For a budget AAA flashlight at $10 that still has a ton of value there is the Olight i3e eos. Build quality isn’t as good, it doesn’t have a clip, but still has a lot to offer.
Olight I3E EOS Black, $9.99
This is a great AAA light, but double your price range.
Titan® Plus Ultra-Compact Variable Output LED Flashlight
ThruNite LED Flashlight--Guiding you through the night! and
Streamlight Tactical & Safety Rated Flashlights & Headlamps also have some really nice slim flashlights. Both these companies are known for standing by their warranty.