...there is a phenomenon known as "Michelin wires" where steel belted radial tires on cars shed very fine short pieces of steel wire and they often end up in the bike lane. I have picked up a couple in my life, and they are very difficult to find in the bike tire. Sometimes they only appear when the tire is pressurized, because they ride within the tire.

You can also end up with something under your rim tape, like small aluminum shavings from when the rim was originally drilled in manufacturing. Those, too, only appear under pressure.

Your best bet is to do as recommended above, then check the spot that seems to be doing this on your wheel or tire.

I confess I once threw away a tire that flatted three or four times, when I couldn't find the problem. It was cheaper and easier to buy a new tire than to keep replacing/patching tubes. And I'm not even a Millennial.