Interesting. I hadn't heard that one. However it's happened before, in the 1980s. Some researchers who were staying at a British Antarctic base over winter were entirely isolated once the last plane left. Months later they starting coming down with colds, despite none of them being sick originally.
Back then epidemiology was a more curious field so this was investigated in some depth. Nobody had opened any new crates or done anything else that could expose fomites. There had obviously been no contact with anyone else. No source for the infection could be identified.
In the wake of this there were a couple of hypotheses which have since been forgotten:
1. Transmission from the atmosphere. There's some other work that explored the idea that influenza could travel long distances in the upper atmosphere whilst remaining viable.
It's known that SARS-like viruses can float through the air because this is how it spread between apartment blocks in a SARS-1 outbreak in Hong Kong. It's also hard to see how it could have spread on the Diamond Princess except via air ducts. Despite being called SARS-2 though, the possibility of airborne transmission was strongly denied at every point by the WHO and other health agencies, presumably because it would have invalidated the rationale for lockdown.
2. Constant low level infection that our immune systems normally keep suppressed. Then you have a bad day, don't sleep well or something else knocks you off balance and this is enough for the infection to start replicating out of control until the big guns are fired up.