Was this a turn around or a pre-flight. If it was a transit check, it is not required to check the tyre pressures with a tyre pressure gauge. If the tyre deflated while passengers were off, they would not have seen it till the aircraft was loaded. If it was a pre-flight they slipped up.
I had a nose wheel reading zero PSI on my pre - flight once. I do agree, with the naked eye I did not spot it. It is however a pre requisite on a pre-flight to check all tyre pressures with a tyre pressure gauge.