There is another possibility no one mentioned. And I wouldn't know if it didn't happen to me.
I come to the bike one day, and the rear is almost flat. "Damn!" I start checking the tire and can't find anything. I inflated the tire, and keep checking more - looking for a sound or bubles (I spit number of times on all suspicious places). Nothing.
So, I start thinking logically, and the only other area to check would be the valve. So, I put more of my saliva on the valve, and what do you know, that's where the leak is.
So, I figure it has something to do with the valve core. So I let the air out, take the core out, and after looking closely I see a grain of sand or something like this stuck to the seal of the valve core.
Just to make sure that was it, I removed the grain of whatever it was, put the core back, pumped the air in, checked - no more leak.
Wouldn't believe myself.
Applying this possibility to the original post. Normally, when there is a good seal, it's not only the spring in the valve that holds the seal, it's also the air pressure. Thos two forces together are more than enough to make sure the centrifugal force is not going to open the valve. However, if there is an obstruction which prevents the tight seal, then it's only the valve spring. Having no cap didn't help either. But having an air tight cap is bad too, because heated air under the cap may push the valve back to the tire releasing some air.
