When a member posts an 'I'm back!' thread after a period of absence, it is correct AH.com etiquette to reply with the phrase 'who are you again?'
This remains the case even if the thread comes after the member has been away for a stupidly short period of time (see AH.com Revolving Door).
The only exception is if it is a member who produced a widely respected work and then mysteriously vanished for a long time, when it is more appropriate to say “(name)! You're alive!”