Carlo's the rat, Chase has given us hints to that recently. He's always late, he makes it an artform. Key latenesses...

1. When Bobby breaks the news that Vito is dead who's missing? The guy assigned to kill him by Silvio, Carlo! Was probably informed by the FBI that Vito was killed. He knew before he walked in.
2. After Chris was killed and everyone is in Tony's room, who's missing? Carlo!

Key Clue...

Before the murder of Fat Dom Gamiello, Silvio was complaining about rat turds. After he disposes of Dom's head he asks Silvio about the BBQ at Sheepshead Bay and if it's still a go.

"When my time comes, tell me, will I stand up?"
Paulie "Walnuts" Gaultiere - The Sopranos