At one point a detective is giving Captain Culpepper an update on the treasure hunters & their locations.He noted that Sylvester is on his way. How did the police know about him at all.They were monitoring the core group who were at the accident site & anyone who joined them along the way. Mrs.Marcus called her son on a pay phone which lead to him becoming involved.The police could not have known who she was calling.None of them overheard the conversation & there was no way the phone was tapped.
This has been discussed before, or something along these lines.
The conclusion was since the original participants had been identified and the general location of where the money had been hidden was also known, further investigation divulged Mrs. Market had a son in the area as well, so he was under surveillance as well.
They saw her making the phone calls and it was just a matter to sort out who she may have been trying to call, and if they had Sylvester under surveillance by then, deduced she had contacted him.
Interestingly enough, there was no scene of his being followed, such as the police copter which appeared when Don Knotts picked up Phil Silvers.
It was discussed before and if one recalls, the police at Smiley Groogan's crash scene (Norman Fell) took down J. Russell Finch's name and address and where he works, and that he was traveling with his wife and mother-in-law, Mrs. Marcus. Finch related that if they called his number, they can reach his secretary. It is no doubt that the police covered all of these bases and then some, and found out that Mr. Marcus had a son living in Santa Rosita.