Conspiracy Theory: Tony had worked for Michael Scott previously...

Tony was quite quick to leave after one day. Some might say it was due to Michael's management style, or trying to lift Tony up on the table. Maybe.

My theory though is that Tony was a former IT guy, either 'Fatty 2' or 'Fatso'. This would explain why he appears in Threat Level Midnight as the piano player. He had no time during ONE day to film this scene. And I strongly doubt he would've came back to film after the events of his ONE day.

Tony is a former IT who worked for Michael Scott previously. The two IT guys in question were never known by name, just their weight. They were big guys, Tony is a big guy.