27 years of being a copper in California.
CVC Section 27605
No person shall own or operate a motor vehicle painted in
the manner described in Section 40800 to resemble a motor vehicle
used by a peace officer or traffic officer on duty for the primary
purpose of enforcing the provisions of Division 10 (commencing with
Section 20000) or Division 11 (commencing with Section 21000)
pursuant to Section 40800.
CVC Section 17604
When a motor vehicle, painted, as required by Section 40800,
and formerly used in the enforcement of the provisions of Division
10 (commencing with Section 20000) or 11 (commencing with Section
21000), is sold to any person and is used for purposes other than law
enforcement, the vehicle shall be painted or partially painted by
the seller or agency formerly using such vehicle so that it will no
longer resemble a vehicle complying with Section 40800 and any
insignia or other marking of the vehicle identifying it as a traffic
law enforcement vehicle shall be removed by the seller or agency
formerly using such vehicle before it shall be operated on any street
or highway, other than to have the vehicle moved to be painted or to
a place of storage.
I once put a citation on an unmarked department vehicle, big Crown Vic / Interceptor with interior mounted emergency lighting, etc. to mess with my buddy while he was having lunch somewhere. He got me back by going 120 MPH one night in that same vehicle coming back from county jail ... a white knuckle ride.
So in general if you don't have a black and white vehicle (Best Buy Geek Squad notwithstanding) and don't have a pushbar, auxiliary lighting, an antenna farm, etc. you probably won't draw attention. Back in the days when the Crown VIc / Interceptor ruled, lots of "grannies" drove the Crown Vic Ltd. / Mercury versions. Now, all the soccer moms, etc. drive Explorers so if it's not B&W, pushbar equipped, etc. you'd stay under the radar.
Just owning a PIU new or auctioned off by a department is perfectly legal ... it's just a vehicle. I looked at a couple before I bought the XLT. I liked the handling, the power, and I could get guys in my department to help me trick it out but I needed the convenience of the 3rd row seats (too lazy to do the conversion to add it in), the big screen with Nav, etc.