It's not too unusual. I have some caps that will get a few volts across them if you leave them charged, then briefly discharge them.
Capacitors are essentially 2 conductors seperated by an insulating layer.
WHen you charge them, you place a tension acrosss the insulator (caused by an electric field) which tries to pull the excess electrons from one side to the side with a deficiency of electrons.
Some of these electrons are pulled right through (this is leakage in the capacitor). Some get pulled part way through.
The ones that get pulled part way through can't get out of the insulator when you briefly discharge the capacitor. Now that the force pulling them into the insulating layer is gone, they slowly return to nearer the surface.
This "stuck" charge appears to com from nowhere to re-charge the capacitor. In fact, it is part of the original charge that couldn't discharge quickly.
I discovered this effect in capacitors used to hold a charge in camera flash units. I thought I was mad, geting a small tingle from a capacitor I had already discharged.
The effect is much stronger (apparantly) in the capacitors used to in microwave ovens. The charge they can re-build up can be enough to kill you. Safety precautions when handling these capacitors includes keeping a shorting strap on them.
