I was planning on using them in an understaircase pantry. Put out very little light. If there is not much light in kitchen, when open pantry door, they would not come on. Worthless for intended use. Don't think would be much use in a closet either. They are going into a garage sale. Maybe I can get 75 cents for them, but will probably not get anything.

My wife laughed at me:"Why would you want a light that comes on when there's light?"

I was wondering how it didn't just stay on after the door was closed, once it created it's own 'ambient light'.

These do work, and in some situations may be just what you need. But I found the ambient light required is too high for most situations. For example, in my spice cabinet, the kitchen lights have to be blazing before this puck lite comes on inside the cabinet. I've moved it around and re-aimed the sensor, but it just requires too much light to be effective.
As to the second question, you can create enough reflection in a tight space, such as a drawer, to have the light sensor keep itself on. But that's not usually the problem.
Bottom line: in most cases this application would be better served by a motion detector, rather than an ambient light detector.
Rite-Lite does make small magnetic switch lites that work very well Rite Lite LPL632 Drawer Light, and are perfect for cabinet drawers.

I put these in my truck box, just for some light if I need to get into it at night. Don't fool yourself and think that these will have the light output of a couple of headlights. They aren't that bright, but they give plenty of light for rummaging around in the bed of a truck. I installed 2 of them, and have the 3rd for something else, just not sure what yet.