The key point here with respect to security at the restaurant is made by FFA:
Originally posted by FrankieFiveAngels:
Sollozzo doesn't expect the Corleone's to know where the meeting is being held, which is why he wasn't that worried about security.
Remember, the Corleones didn't find out where the meeting would be held until virtually the last minute. Sollozzo presumably thought it was
still a secret.
His "man" in the restaurant was simply an added precaution. Sollozzo truly believed he was completely safe while being guarded by a New York City Police Captain.
