Air pressure loss / Valve caps??

I swear I'm not crazy. I left my valve caps off two days ago after airing up all four tires (went off-loading so had to deflate to 20psi). I have stock setup and the last two days the air pressure was fine. As I was driving on the freeway today, I heard a loud whistling / hissing sound coming from the rear passenger area. The air pressure alert came on and notified me I had only 9psi in the back right tire. Hours and hours later, same pressure. Had to drive about 1.5 miles to get to a gas station and during that drive, the pressure raised one psi, as is typical during driving, in the affected tire. I filled the tire with air and drove for about an hour. The tire maintained its pressure and is sitting over night to confirm no loss of air.

Is it actually possible that, at freeway speeds, a pebble or rock got lodged in the valve and the hissing I heard was air being released to 9psi? I had always heard that was urban legend and the stock valve caps don't actually offer any real protection… any help on this would keep my head from spinning.

 

The answer is no, Brendan, you idiot. There is a nail in your tire.