Let me suggest another hypothesis, based on the pressure you are using and the slowness of the leak. You have a small hole in your tire's carcass. At high pressure, the tube comes through this hole comes in contact with the road surface and punctures. As the pressure reduces the tube partially withdraws from the small hole in the tire but not completely. This partially closes the puncture and you continue riding on what appears to be a reasonably inflated tire.

When you line up your tube puncture with your tire, also check for a tiny hole in the tire carcass, in addition to a sliver of glass or metal embedded in the tire.