Something you can do. Take the tube out and figure out how it is arranged vs the tire. Inflate the tube and stick it under water. You should be able to detect a slow leak by finding where the bubbles come out. Then knowing where on the tube the hole is, go find the same spot on the tire and see if the offending object is still there. I have had a few flats that were caused by those tiny steel wires that are in steel belted auto tires. They are almost invisible and are very difficult to find on a tire unless you know exactly where to look. Sometimes small slivers of glass behave the same way. Of course, you could just pitch the tube and replace it and hope that your luck holds.