You raise an interesting point, DonJon. Logically, if the job the guys were given was to protect Michael, they would have stayed with him at the hospital. But FFC was seeking to make a very dramatic scene, and it was--Michael, alone, visits the hospital and finds it deserted, knows his father and he are in mortal danger, acts courageously and resourcefully...

It also highlights the transition of Mob "values": Clemenza has no doubt whatsoever that Michael would be in no danger going to the city without bodyguards because "Solozzo knows he's a civilian. Later, Apollonia gets blown up because she happens to drive Michael's car. Then, during the Great Massacre of 1955, there's more "collateral damage": Rocco and an associate machine-gun the hooker who has the misfortune of being with Tattaglia; Neri kills Barzini's chauffeur; Clemenza kills the elevator operator and the guy in the car with Stracci...By GFII, Kay nearly gets wiped out because she's in the bedroom with Michael. Sic transit Mob "code of honor."