My bathroom gray tank does the same thing. The Kitchen one doesn't. But, the Bathroom and Black tank handles go into the underbelly and contol valves remotely. The kitchen gate is separate and is right at the connection. So, I attribute it like others have, may be some residual in the pipe that gets jostled to the cap by the next time I'm ready to take it off. At least it isn't sewage. Always smells like soap or shampoo from the shower thank goodness!
Pull-Rite 16K Super 5th
2014 Rockwood Signature Ultra-Lite 8289WS
2011 Chevy 2500HD - DuraMax (Just over 5000 miles now!)
Nights camped in 2014 - 21