GTA5, Are There Still Secret Cops Cars?

On xbox 360, PS3, and I think there's also the FIB Baffalo on a PC.

Is there a similar one for Xbox one?

As far as I know, there's every police car on every platform.

Hmm But somehow not.

Yes there's a black FBI / police car, unfortunately I can't tell you where it is.

I checked again. There's the FIB car in the code, but you can't find it like you did with the XBox 360. Only in certain missions. We can confirm it by playing the following missions mentioned in this post.
http://de.gta.wikia.com/wiki/FIB_(V)