USB and small...........maybe consider the cooyoo quantum/ fenix uc02/jetbeam 1 options. Fed on the tiny(around 1/2 the size of an AAA cell) 10180 battery, 4.2v and maybe 80-100mah ish. Not much granted, but enough to fire out 100lm when needed fast. Lower mode of 10-20lm for longer uses(not talking hours here though). Key chain wise i find perfect, not too big or heavy to even notice its there, yet it is when you need it!
Come in a variety of flavours like stainless, ti, copper to suit pretty much most. Beam wise is a useful all rounder through the TIR optic.
My most recent pic of my cooyoo(couple of years + its been used now)on the left above the SAK alox.
2 mode tisty, tighten a bit for low mode, tighten more for high, undo head and USB charged at around 100mah (i measured).
Here is another pic i have found next to mr vader
These are not USB rechargeable, but still tiny little lights from DQG called the spy and the brass version called Fairy(who knows why!)
These are mules(no reflector) which offer a small wall of close up light.....................nice warm light in this case. Very easy and pleasing on tired eyes searching for lost stuff at night.
Madisons jetbeam 1 heat treated off vinh
Little light pic i have dug out, the cooyoo stainless steel on far left,jetbeam 1, ti in packet and the DGQ lights.
For the last,..maybe year now i have been using the DQG hobi in work on those keys. Simply because of size/weight advantage over AAA of previous uses. They make a good back up, but if regular use will be required, the smaller cells are not ideal.
Good luck in your choice