So would you be OK as a host if your sitter showed up two or three hours early? Most people aren’t usually sitting around doing nothing before a trip, whether as a host or sitter or other type of guest. They might well be busy.
As a telecommuter, I might be having a video meeting for work, for instance, with a bunch of people on the other end. It would be disruptive to have someone turn up. Some telecommuters do work that requires privacy, along with quiet. At home, I even have a sign on my office door to indicate that I shouldn’t be bothered by my husband while I’m in meetings.
Personally, even as a guest invited over to someone’s home, I don’t arrive early. Say traffic moved faster than expected, then I park myself till it’s actually time to arrive. I even made that offer to a host recently when my three+ hour drive went faster than expected. I offered to go get coffee or breakfast if they weren’t ready for me, because I didn’t want to inconvenience them.
In the scenario you described, I’d contact the sitter and ask whether they’d rather I stay at the airport or whether they were OK with me turning up early. And I’d respect their answer out of courtesy. The expectation for sits is that the sitter gets privacy. Why should that be randomly voided early?
Adding this, which I just stumbled across on a recommended thread that’s old: