Maybe the reason McGonagall knew that Dumbledore was behind the Santa Claus portkey is because only the headmaster could create a portkey that would work inside the Hogwarts wards.
Then again, Snape didn’t realize that just from hearing about the portkey. This theory’s probably inaccurate.
Then again, Snape didn’t realize that just from hearing about the portkey. This theory’s probably inaccurate.
Retracting as per pedanterrific’s comment.