It is possible to get a bit of grit in the valve which will then let a small amount of air through.
This is why it's a good idea to keep dustcaps in place.
You don't say the result of the spit test, but it might be worth getting a valve key, unscrewing the valve core half a turn and re-seating it.
You'll not lose much air.
A valve key will only cost a pound or two from an accessory shop.
I have one incorporated into one of the dustcaps.
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