Hi PK,
Not sure if you saw my post, but I had the same thing happen to me recently.
I am wondering if by chance, you were using whele chocks. The type that go between the wheels. I use Roto Chocks and they go between the wheels and spread them apart to lock them rock solid.
It has been surmised by another forum member that maybe I had them too tight. I had them really cranked down hard, the tires had "just" been mounted on the rims and maybe, just maybe, they had not had enough time at speed and temperature to seat properly. Just a guess.
Like you, no leak could be found on my tire, rim or valve stem. They were checked thoroughly - twice in the field, then I took it into Sears Auto to dunk in their tank and no bubble.
It's now my spare. I guess we'll find out next time I have to put it on the trailer if it will hold. the only thing I did not test was having it on the trailer with that high vertical load on the tire.
Let us know if you find out anything more.
Jim