Maybe the fact that he was standing still while we heard footsteps. That's a pretty big clue if you ask me.

Also the fact that he not only noticed the wrist watch that was stopped at 6:15 but also the fact that he took that severed arm with the wrist watch and wrapped it with a wrench so that it would sink in the water and planned to have his dog there as a alibi.

Did none of you watch The Sopranos? Who can forget that last scene where it cuts to black and the audience is left wondering if it was Meadow coming back from the bathroom or if it was a hit team. I think this was an allusion to that. It leaves it up to the viewer to decide his fate. I think the ending was good but…