Building on
prj71 post, I had a similar issue with my gravel bike a few years back. I brought to the shop to get a new tube and the mechanic had time to look the issue. He took the tire, looking at the inside, rolled the tire bead-to-bead , working his way around the tire, and found a small piece of glass imbedded in the tire. It was so small you wouldn't feel it rubbing the inside of the tire with your fingers, and it wouldn't cause a flat right away ... but hit a bump at the right (or wrong?) time and the glass shard would put a small hole in the tube.
You can try the soapy water trick with the whole tire too ... spray soapy water on the tire and inflate the tire to max pressure, and the bubbles will show you where you have a hole in the tire, and that where you look for a small shard of glass or other tiny sharp object.