Hi Andy;

The fact that you can pull up on the cap and see a gap would indicate that the cap is not fitting tight enough.  The fuel caps on my Cessna's have always fit very tight. I would think this is the most likely source of the water.  The rubber gaskets do wear out, and the metal tangs that fit under the filler neck can get bent so that it doesn't fit as tight as it should.

It might also be possible that some water could enter through the vents.  On a 172 there is usually a vent in the cap opposite the under wing vent tube.  As the fuel level drops it will draw air through the vents.  From your post it sounds like you are checking right after flight, so it is unlikely the water is from condensation.  I am assuming that you are getting all the water out when you check the sumps, so it isn't carried over from the previous check.

Hope this helps.

Jon