I don't think they would have to know his schedule. Since USSS is "under" Homeland Security, I will assume they have tracking abilities of the SS agents. The agents go before President Trump to clear (secure) the way. By viewing SS locations on a map, it would be made clear they were on the golf course. They would also be aware of the time involved approximately of each hole (based on historical data) and duration of the round. It would be quite easy to determine approximate location within X minutes.