Fire-Message to Regulus
Snape apparated into his office and spent a truly frantic few minutes checking all of the castle's defences. When he'd assured himself that the castle wasn't yet under attack, he spared a moment to send a fire-message to Regulus:
This is a diversion. Get Lupin, Longbottom and Pettigrew out of there, and deploy yourself and them to guard the Hogwarts Express. I'm at Hogwarts in case that's another target.