I had a similar problem too, and only in the Summer after a road trip of say 50-60 miles of steady highway miles.
I was using radial tubes, and running TSL's on the stock rims. I had good flaps and they were Goodyear tubes that were quite thick and heavy-duty.
On the first flat, there was a small tag inside the tire that was glued to the inside. It came loose I suppose from the scrubbing of the tube, wadded up and rubbed a hole in the tube.
The second flat showed no sign of any foreign object, and by the time I stopped, the tire had been flat long enough that the tube was torn pretty badly. Not sure if it was from the actual blowout, or from the driving a little too far on the flat.
The third (and last) flat was similar to the first, but again, no foreign object I could find, just a very small localized spot rubbed on the tube and no real reason I could see.
After this and enough of replacing expensive and less common to find tubes, I ordered a set of Marsh wheels and now run tubeless. I keep a plug kit in the toolbox, but as of yet haven't needed one, and I can run 50 all day without worry.
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