Not an LEO, but a scanner in a vehicle is illegal in NYS. It's a VTL violation. We had a cop in my dept summonse a kid on a bike with a scanner that was mounted to it.
A cop would be well within their duties to arrest or summonse a person violating this law. Most cops I know would let them go especially since around here most of the volunteer firefighters and EMS types are the ones doing it. When someone lies and without a volunteer FD/EMS affiliation, they'd probably at least get summonsed, if not arrested. Also, it at the very least would cause the officer to do a formal field interview of the subject.
In Nassau and Suffolk Counties they are very lax with the VTL's as they relate to vollies. Many vollie FD/EMS in some way, shape, or form often violate the VTL in regrads to the blue light they display. You're allowed one blue light in your vehicle. Lots go all out on lightbars, tailflashers, wig wags and rear deck setups that would make a highway cop jealous.
There was a cop in one precint who banged enough VFD guys from one department that they all had to get rid of their extra lights, gizmos and gear, and when he left the precint, it all came back.
If someone tells you it's a HAM setup and a PD freq comes blaring out, then obviously the officer's suspicion has been aroused and has probale cause/ can up the tiers of inquiry to get to the bottom of the situation. COuld this be a guy casing houses and PD's response to automatic alarms? Checking banks for PD presence?
We has a rash of robberies in Nassau of jewelry stores and the perps were using scanners to remain appraised of PD response and subsequent evasion.
It's up to the cop what they want to do.