Wait until night (after 8pm game time) when the targets show up.
Approach the target until you get a message about HIDDEN LOCATION and then examine the modulator to use it on the target. (9/1/12 - I get no message. I found a convoluted method that seems to get the updates consistently. Crouch, walk up to the NPC and Hail them. Back up to the waypoint and use the device.)
Listen in on Augurer Phineas in South Qeynos across from the mage tower bridge at ( 652, -19, 142 ) /waypoint 652, -19, 142. I could use the device here; ( 639, -18, 119 ) /waypoint 639, -18, 119.