Assuming Presta:
Because or reasons, I use the valve caps. And occasionally these can be on tight enough to loosen a removable valve core.
If that happens, IME, the core will usually continue to back out entirely. But I suppose it’s possible that it’ll simply back out a little bit, enough to leak.
Or maybe you’ve forgotten to set the knurled locknut?
That too can give an unpredictable result.