I rode my bike 2 days in a row, left it home the next day and the rear tire went flat by that evening. I inflated it last night to check for leaks or punctures but it's still inflated, I always check my tires before riding and I had no problems during the two days I rode. The tire went flat while it was not in use and the pressure is still the same since I last inflated it. I don't see any visible reason why it would deflate so much so fast especially since it seems fine now. I'm concerned if I need to replace the innertube or if this was just a fluke.
- Simple puncture that's somehow letting air out intermittently. Hard to imagine the physics of how that would work, and even it was happening somehow you need to replace or patch the tube as a practical matter.
- Valve problem, as Eric Nolan points out in the comments. For example, the little seal inside a presta valve core may be torn or defective and shifting around as the valve is opened and closed.
- In a tube with sealant, the sealant is depleted or otherwise becoming unable to seal a prior puncture. Any tube or tubeless tire where sealant is used will eventually reach this point if the sealant isn't replenished.
So, if you want to experiment/gamble, you could try replacing the valve core if it's a Schraeder or a removable core Presta tube. In doing this you might be able to simply see the torn seal, which gives you a pretty good guess that's what the problem was if so.
If there's sealant of any kind involved already, it's very likely you just need to add more.
Otherwise, replace the tube.
I had a puncture once that sounds similar to this.
I'd go for a ride and the next day my tire would be flat. I could fill up the tire and let the bike sit for days without any noticeable air loss. Then I'd go for another ride and within 6-12 hours, the tire would go flat again. I'd pull the tire and tube off and look for punctures and not find anything. I couldn't find anything in the tire casing and no holes in the tube, even if I inflated it and submerged it in water.
After several attempts at removing the tire and tube and searching for holes and punctures, I eventually found a tiny sliver of metal that had just barely punched through the tire casing. It was so small that it felt like a bump in the casing when I ran my finger across it. I couldn't get it out with my fingers or even tweezers. I had to get a knife and cut into the tube a bit to create enough space to get tweezers on it. Once I pulled it out and looked at it, it was no wider than a hair and maybe 1/4 inch long.
Once I found the sliver, I inflated the tube and submerged it in water again, but this time I overinflated it by quite a bit, so much that I was afraid it was going to burst from the pressure. Voila! Bubbles from a bunch of tiny holes.
I put in a new tube and was good to go.
I think what was happening was that the punctures were so small that they just didn't really leak without the metal sliver sitting inside of them to hold them open. And the sliver wouldn't create a new hole until I went for a ride and put some pressure and friction on it. But once it pushed through the tube, it held that new puncture open just enough for a slow leak.