Small but nagging question

When Shaker tells Tom that someone will be watching him at the gym when he dives in the pool, was he telling the truth or just bluffing to keep Tom on his toes? It's been a while since I last saw this film but if I recall correctly, all the kidnappers are there at the hideout when Shaker is giving him instructions and unless one or more of them was gone, it seems unlikely that there would be another accomplice keeping tabs on Tom at the gym but otherwise having no involvment in the story.