Maybe a different carrier with different coverage?
I'm also wondering if Find My will save the "find", including GPS coordinates at the time, and report it out when it has connection?
Find My doesn't require the passerby to have connectivity _at the time it passes by_. They record where they were when they saw the lost phone, and whenever they get connectivity back, they report that to the owner.
They didn’t, they were passing by, and eventually headed to an area with coverage.