Oh, they certainly do have secure comms. All USSS Presidential Protection comms are routinely encrypted. Today, almost all voice comms were Secure Project-25. Until recently they used 12kbit CVSD encryption.
What you can monitor is supporting local and state police agencies that help with motorcade and area protection. Military and Civil Aeronautical channels are also helpful.
No, not usually. From my own personal experience from my time working with the USSS, I can tell you that most comms dealing with POTUS, VPOTUS, etc, are encrypted using techniques that are way beyond what any scanner could deal with. A lot of money is spent making sure that those comms can't be monitored. And the USSS has far greater concerns than those of us with access to scanners.