The Lumintop Tool AA 2.0 is an AA/14500 light that comes with a rear clicky switch, but it can also be purchased with a magnetic tail cap. Small enough to clip in a shirt pocket, but puts out a lot of light when needed. I run mine with a 14500 Li Ion rechargeable, but if you are not into that AA cells will work but not be as bright.
I agree that if you want to light up a larger area like under the body or in the engine compartment a COB light would be good. If you don't want to go with the el cheapo ones, Eveready sells a couple combo lights with both a regular bulb and a COB sidebar.
My understanding is that some leaks show up better under UV light, so you might want to look into getting one of those too. And we can't forget the venerable Photon lights. They have an accessory that allows one to wear the light on a finger, which would be useful for close quarters especially around and behind things.