Originally Posted by MarkSF
Drain plugs love to leak. Try some Vasoline around the plug next time. If it is the type with a proper O ring, what state is it in?
Next time, determine whether the water is salt or fresh. It could easily be some rainwater that collected somewhere, and moved while under sail.
Another trick you can try to find a leak is : fill the boat with water when it's on the trailer. Not too much though, it is heavy stuff and you don't want to cause structural damage to the boat or trailer by adding a ton of water.
It seems that the worry about a leak might have been premature. I went out again yesterday and after 2.5 hours, only a couple of drops came out of the drain, rather than a couple of cups.
So, either the drain plug wasn't secured the first time, or as you said, there might have been some rainwater left somewhere. I drained some, and thought I had gotten it all, but maybe not.