Psh, this should be easy. She sets a cuddly little VI out to crawl for anything that looks like 'Quarian not around other Quarians' on whatever security feeds it can worm its little body into. Not a huge chance of pinging anything useful, but it's free and it's fun.
"Exactly what I needed, thank you, Warhead." The compliment doesn't hurt, either.
Terentin's main plan now is a little less fun, but just as free, and even lower-tech than the VI crawler. She tosses out an auto-email to Sal'Poma's address, coded to resend as constantly as local spam filters will allow. The email leaves behind a snail-trail in every part of the Citadel net that it gets routed through. Now they should be able to just follow it, at least to her general area. Ideally they'll catch the girl crossing from one router service to the next, but that's relatively unlikely. The end product of all of this hovers over her wrist, a single blue section of the Citadel in a horde of orange ones. A little timer ticks down until the next email ping in about four minutes.
"It's nice working somewhere with public wifi for once. I'd say there's a 90% chance she's in there somewhere," she says, indicating the blue zone. "She might have just dropped her tech, though."
(The email itself, for listeners at home, is a pretty standard Alliance recruitment flyer!)