Tests can be false negative, false positive, there can be sample contamination giving positive results and sample contamination giving negative results. Rest easy, you loved your dad and it was his time.My Dad passed away 10th January 2020 of lung failure in hospital in Florida. He fell ill on 18th Dec 2019 aboard a Panamanian cruise which departed from Ft. Lauderdale. Family members are still insisting it was COVID-19 related, but the timing is more than a bit off (his illness being so early). As Dad has not been interned yet I'm tempted to try and get a test done post mortem as un-resolved I'll be hearing about it for the rest of my days.
Dornole's note made me think it is possible he was "patient zero" but unlikely as to my knowledge none of the ICU staff were infected (and no family were). How to make a tough situation tougher for sure. Dad was 76 and still working. Issues, like we call accumulate, but I has him pegged for 85. Treasure every interaction.
There could be no "Patient Zero" and the odds are there probably isn't one. People used to say that Gaëtan Dugas is patient zero for HIV in the US, but they found out this wasn't true. He was dubbed "Patient-O" as in the letter "O" for "Out of California" but this was for privacy reasons and was corrupted into Patient Zero. He was just one very active guy that could be easily tracked and wasn't from the US.
It's pretty common for scientists to invent and misrepresent/distort evidence to support their expectations. It's a normal human failing. Most conscientious scientist don't even know when they are doing it.