It really isn't. When you know where they started from, and what direction they were supposed to be heading in, then even without knowing how fast they're supposed to be going, it's literally as simple as dropping out of FTL at regular intervals behind the sleeper ship and pointing a telescope in the general direction you're going until you hit the sleeper ship's light cone. What other posters have suggested about potential technical limitations relating the nature of the FTL drive and/or logistical problems with actually doing a pick up make sense as blocking issues, but finding them to begin with is a solved problem. Like, this is basically "where are Voyager 1 & 2 right now", and we actually know exactly where they are right now because we're still picking up their radio signals, powered by a 249W generator (less power than used by a typical modern PC!), from over 136 AU out, and a sleeper ship is going to be way more visible than that.