The parking sensors were an add on stand alone option and the first Model S's began appearing on or after August 2013.
Then in Mid 2014 (say June or July) the parking sensors were removed as a stand alone option and were added to the Tech Package.
Then in April 2015 the parking sensors became standard equipment and were no longer an option.
That is pretty much the history of the parking sensors.
The parking sensors appear as round button sized mild indents on the front and rear bumpers and the nosecone.
When the Autopilot hardware package was introduced 4 more sonic sensors were added, two on the front edge of the front bumper near the wheel wells and also two near the rear edge of the rear bumpers along with a centrally located camera in the inside of the windshield.
Of note: some early Model S owners have retrofitted non-OEM Tesla parking sensors to their early cars. I recall a thread or two regarding this.
Lastly, for some folks, the Model S is challenging to park consistently with accuracy, especially when backing in. I suspect that the parking sensors would be of great assistance to those drivers. The Model S, is, after all a wide car, (about 2" wider than my Volvo X60-R-Design. It is for this reason that I have retrofitted my two 8' wide car garage doors with one 17' wide garage door thus eliminating the central post which was between the older 8' doors... it was TESLA-FIED!:tongue
