You can go several routes with this one.
On the surface, when stopping people with scanners, it definitely piques my interest. I will more than likely ask why they have it; a simple, polite answer from them usually resolves any interest I have. If things aren't adding up or I think that they are using the scanner to "ambulance chase" police, fire or EMS, thats where the big problems start. People have been known to try to chase the calls versus just listening. I am not beyond putting someone in jail for parking on the same block as a fire/ems call (see Texas Transportation Code) if they push it.
Why do you think it's important to know why they are carrying a perfectly legal device? Do you ask them why they have a cell phone or a camera or an iPod? What pretext, excuse, or reason do you use to justify arresting a person who is sitting in a car, presumably legally parked? People have in deed been known to use a scanner to get in the way of police, EMS, or fire operations, but I'd think one would be hard pressed to prove that if they were sitting in a car a block or so away. I've seen fire buffs creep beyond police lines a fire scenes, but the most the police do is shoo them away back to the other side of the yellow tape.
If we start going down the criminal offense route, if you are found to have it during the commission of an offense and it can be articulated that you used the scanner to further the offense, then I could add the offense of Penal Code 16.01 Unlawful Use of Criminal Instrument which would be one offense level lower than the offense you committed. Additionally, if you start interfering with my public duties, we can go the Penal Code 38.15 Interfering with Public Duties route which would be a Misdemeanor B, up to 180 days in jail.
If you want to scan, keep it legal and don't be a stalker.
Again, I have to wonder how sitting in one's own, presumably legally parked car constitutes the crime of Interfering with Public Duties? If the car is illegally parked, isn't the easiest thing to do to ask the driver to move it? Under Texas law, is a car that is stopped, with the engine running and driver sitting behind the wheel considered parked? Or is that considered "standing"? I'd suspect the latter, since if the person were DWI under those exact conditions I expect Texas law would consider it "operating" and not "parked".
BTW, before you start the "You're anti police" crapola, I was police for over 20 years, even if only part time, including several years as a Sgt. If one of my officers acted as you describe, he'd have a lot of explaining to do. I always thought my job was to serve the law abiding public, not harass them and make up crimes.
I was recently on IH35 near Lorena at the picnic area. Previously, I had been traveling south on IH35 around mile marker 318 or so. All of the sudden, my Beltronics 995 emitted a "Laser alert." I looked up and observed a Lorena SUV hidden in the tree line. For some unknown reason, I innocently reached for something that fell in the floorboard of on the passenger side. I proceeded to the aforementioned picnic area south of this location. In about 10 minutes, I observed the Lorena unit leaving the interstate, heading into the picnic area, and making a beeline for my automobile which was parked on the north side. I was sitting outside at the picnic table with my scanner while smoking a cigar..
What brand or brands do you like? I'm a big H. Upmann fan, but I also like Rocky Patel.
I'll also add that I've driven with a scanner in my vehicle in a number of states that have restrictions including New York, Florida, Kentucky. I've never been stopped in any of those states, nor had any encounters with police in any of those states. I think the key to that is the same as carrying a concealed firearm. Don't draw attention to yourself. I've even scanned in airports without drawing the attention of the
Too
Stupid for
Arby's crack inspectors or the airport police. Low key is the key.
BTW, if you have CCW permit in TX, you do have to disclose that you are carrying if stopped while driving.