Possible scenario is:
Wire or nail or whatever pierces the tire & tube and cause the puncture but because its a long piece of wire it gets pulled out before you stop.
The hole might be small. The pressure of the tube against the inside of the tire might be holding it closed.
I'd be surprised if it doesn't slowly deflate
Did you do the hold "inflate tube and submerse it in water" trick to look for tiny bubbles bleeding out anywhere?
Stew
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The older I get the better I used to be.