I have a very similar problem. I didn't know what was causing it but every so often I have to dump a little coolant into the car. I noticed it is on the ground infront of the rear passenger wheel. One time I saw a very slight bit on the drivers side infront of the rear wheel. Figured if I saw that again I'd get it looked at but never have.
If this is the consensus that a new cap is necessary I'll order it too
-Matt