So I was out riding yesterday when I heard the definite sounds of a leak from my rear tire. Since I was in a place where I really couldn't stop I rode on a little ways. By the time I stopped, so had the leak (?!?). Today at lunch I went out to my car to (briefly) look at the tire. It hadn't lost anymore air, and when I tried to put air in I couldn't get the pump to register over 40 psi but I can tell there is clearly more air than that in the tire.

My question is...uh....what the hell?

check your tire guage...and your hearing

It's possible the inner seal of your tube's valve is failing.

That's what I'm thinking. Did you try to reposition the pump chuck on the valve?