Was planning on a road bike ride yesterday morning. Got to the bike and the tire is flat. I went to the gym instead. When I got back I pumped up the tire and figured I'd fix the flat at night.

I go to the bike 24 hours later and the tire is fully inflated.

How does that happen? Over my 20 years of mountain biking / road biking I've seen this a handful of times. But never really understood why.

I'm guessing it has something to do with the presta valve.

The only rational explanation is that the presta valve was stuck slightly open. I would replace the valve core if removable.

There is an additional possibility, which is that the tire simply has a slow leak, and it had been significantly more than 24 hours since the person checked it, or inflated it.

I suspect that everyone with a stable of bikes, or who rides infrequently, has this happen from time to time.

there was probably a piece of dirt or sand grit in the valve when you last aired up the tube... re-airing it moved the grit out of the valve.

i always cap the tube valves to limit the dirt intrusion.
it can still get in.... sometimes from the pump side of things.