As @Mapperz said road data can include easter eggs to prove copyright violation. (I am aware of a case where this was used to show violation but I cannot find it now after searching.) However, this does not help with polyline data.
You can approach this using digital watermarking. There are a large number of papers on this. I have two examples which you can find on IEEE or ACM.
Michael Voigt and Christoph Busch, "Feature-based watermarking of 2D vector data", Proc. SPIE 5020, 359 (2003); doi:10.1117/12.476815
Michael Voigt, "Watermarking geographic vector-data using a variable strip-size scheme", Proc. SPIE 6505, 65051V (2007); doi:10.1117/12.704557
I would recommend Google searches for "polyline digital watermarking", "GIS digital watermarking" and other variations. Secondly, if you follow citations to the Voigt papers you will find more. A large number of them are written by Chinese or Japanese researchers so the English translation can be difficult and the heavy maths involved make it worse.
Being simplistic, the watermarking techniques involve moving some of the points in special ways such that there is a signal in the points which can be detected if you know how to look for it but the changes are less than the noise error. Some of the techniques are "blind" - this means that the watermark can be detected without the original data.