Probably a very very slow leak from when I drained it last fall... There's lots of sediment inside that I hear knocking around so I try to drain it yearly. Guess it was so slow I didn't notice when I closed it back up. I've watched it for a couple minutes now and it hasn't actually dripped down yet. See what I mean though.... no skills with this sort of stuff.
Yeah, I know what you mean about that sediment. Down here, in limestone country, I've had so much limestone sediment build up in the bottom of mine, it reached the bottom element (electric water heater). That drain valve looks like the one on mine. If so, once you've got the water drained, you can take the top off that valve. Once you do that, you can clean the sediment that will be laying under the washer, where it closes off the flow. Also, I've seen those valve leaks stop after awhile--my theory is more sediment works its way down to the valve and blocks the remaining flow.