Last Friday on Ride from Seattle to Vancouver and Party (RSVP), we got little rain going on from around 10AM and when I got to Chuckanut, some 10 miles to Bellingham mid point got a flat when climbing.
Ok, that is life and didn't find any external rock chip, glass anything after trying to look the wet tire. There was a single puncture in the tube, so some nail or some thorn????
Put on a new tube - only had one with me - and rode close to the bag stop for the day.
A block before getting there I notice I am losing pressure again on my back tire...
Just walked to the bag stop- I had another tube in my bag - this time was a defective tube that got a hole in a thin section,
REI was two blocks from there, so got extra 2 tubes.
No issues on next day that was much more pleasant with no rain and nice cool weather for bike ride.

I got a previous flat on the road 2 years ago commuting home and another this year with a defective tube close to the presta valve - this one I notice in the garage.