If you're talking about IMAGE it was known where it was NASA just didn't know that it was still transmitting. IIRC the discoverer actually used the orbital elements to determine what satellite he was seeing. They still had an active and accurate TLE for it they'd just stopped checking because NASA thought it was dead in space after the eclipse a couple years ago failed to reset the broken power supply. [0]

[0] https://skyriddles.wordpress.com/2018/01/21/nasas-long-dead-...