Mystery flat

I went out to the garage today to get the trike out for my first February ride only to find a flat tire!  I'd aired the tires prior to my last ride a few days ago, so I figured I'd picked up a nail or rock chip during that ride.  I wheeled the trike to the front of the house to catch the warm rays of the sun during the repair.  After removing the tube from the tire, I inflated it but wasn't able to find the leak: even after submerging it in the bath tub.  After reassembly and inflation I went out Airport road, stopping periodically to check the tire--no discernible leakage.  There was just enough of a headwind to make the wind chill quite uncomfortable, so I only rode out about three miles.  The return portion of the ride was quite nice, as the headwind-turned-tailwind was negated by my speed and it was as though I was riding in a bubble.  The tire held air throughout the trip--no idea what I'll find when I go out for my next ride.