At the small water district i work for, we have a person that only does locates and construction meters sets. As Benbo mentioned, The gas company does their own locates; and the telcom/elec uses utiliquest.
I have seen some situations where the utiliquest guy became confused and located our waterlines as electric lines. You think the valve can with the 3' diameter concrete collar and the blue paint that says water would be the give away that it's not electrical. I've noticed that the locaters will try to out wait each other to be the last one to locate.
I've had a project that required the gas locater to stand by for about 36hrs ( i had 3-12 hour shifts come in). There is a new law in california that anytime work is done near a high pressure gas main, the gas company has to have a rep on site during the entire construction. My crew started opening up the ground for an emergency 24" watermain replacement, when the gas guy drove by. The locater didnt care, because of the OT, but his supervisor was really pissed. after 36 hrs, he looked pretty bad.