Ron Dvorak pointed out in a private message that my response was not entirely correct. And he's right.
Mobile units (smartphones, tablets, or anything else that connects to a wireless network) send probes to all allowed channels, looking for access points. This behavior is supposed to continue after the MU has selected one SSID and connected to it. But that's not always the case, and depends on the MU.
If you have MU's that frequently send probes in all channels, even when connected to an SSID, then locationing may be able to triangulate the position of the MU. But if it does not (many MU's, when connected to an SSID stop sending probes to save battery, unless something happens, like a fading connection), then you will not be able to triangulate. Unless tou have additional APs acting as sensors.
So, if locationing is important for you, and you want to make sure it works, no matter what, the recommendation is to add a couple of sensors.
The topology of your deployment is also important, so EMC includes a Heat Map that shows how well prepared is your network for locationing.
Bear in mind that, since location is calculated, it is VERY important to correctly scale the map and to mark all walls, doors, windows and other obstacles to the signal, with their correct construction materials. EMC will take all that into account to calculate the atenuation suffered by each AP's signal when traversing those obstacles, and better estimate the distance between the AP and the MU.