"How did you know it was me?"
This is what Minerva says when Dumbledore reveals that he knows she was there...
"Fancy seeing you here, Professor McGonagall." … She looked distinctly ruffled.
She shouldn't have been so surprised if she was registered, now would she?
Yes, I know she was registered when Hermione checked, but that was a decade later.
Was Minerva McGonagall an unregistered animagus in 1981?