To me, 11x21 seems like it will mostly take place away from GSMH. My theory is Derek suggests he and Meredith go on a road trip/vacation away from GSMH. He's trying to fix his marriage, so it makes sense that they would go for some alone time. The only people we know for sure were on set during the filming of this episode were EP and PD. Everyone else was radio silent/no BTS pictures. Also seems like they were on location almost every day. Also, this season parallels S4 in many ways. Derek wanted to take Meredith on a getaway then, so maybe they actually go. They can't go on a getaway without major drama though, since this is Shondaland.
My guess is they either witness or come across an accident, then decide to help out. PD was on location. They would have shot the accident on a green screen most likely, and if he was lying there injured, as someone mentioned, I don't think he would have needed to miss two races for filming. I think he's actually in the field, saving lives.
Someone said the media was on set for 11x21. That's actually wrong. They were on set for 11x20.