Bikewer beat me to it. Slow leak hard to find. I've picked up steel belt from a car. I've had it happen three times. All 3 times it was hard to find. The most striking happened on a long ride. Tire went flat, could not even find the leak in the tube at first (Got lucky a store let me use a sink in back, with total immersion I found the leak). Checked the tire and found nothing. Replaced the tube. Pumped it up. Rode about 50 miles no problem. Put my bike in the back of the car at the end of the ride. Came out next morning and the tire was flat.

Took a good half hour to find the piece of steel belt, which may have been in the tube for a long time. I'm convinced the original flat happened when going over train tracks about 2 miles before the store (which was a schedules rest stop). This drove the belt through just enough to create a slow leak. But once off the track the steel belt slipped back into the tire, leaving nothing to see or feel.