At first I couldn't figure out why the same mitigations didn't work this time, but then I noticed the pattern of who was getting infected and when.
6 people drove back on a 4 hrs road trip after 4th of July. The front passenger had sniffles, but tested negative. They tested multiple times in a row to be sure.
A theory: front passenger caught Covid during the trip and were on their "day zero, negative results" day.
Driver and passenger directly behind them had their "day 0," two days later.