His radar was probably just "on".

Level 1 Peace Officers have statewide jurisdiction basically for misdemeanor and above crimes. There is case law that would prevent an officer from enforcing traffic offenses outside his jurisdiction unless, like has been said it was some more serious situation.

A deputy sheriff can enforce all laws anywhere in the county. So a JeffCo deputy could conduct traffic enforcement in the city of Arvada, for example. They usually don't make a habit of it, though.

Fentonite's answer is pretty much right on.