This object was "lost" for some decades. In October 2014, the Palomar Transient Factory (which was looking for near-earth asteroids that might hit us) recovered it. That was apparently enough to get the Space-Track folks back on it again.
The near-earth asteroid folks stumbled across it again in November 2017, so we now have a pretty good orbit for it.
and links to https://www.projectpluto.com/pluto/mpecs/63039c.htm
The spacecraft is in HEO (high Earth orbit) at something like 100,000 km; a quarter of the way to the Moon, so it was
- hard to track via radar
- subject perturbational evolution due to Sun, Moon, Sunlight, etc.
Also, this answer to Suppose Zuma wasn't “lost” — what would it take to detect it? describes the "rediscovery" of a satellite in a 1000 x 46,000 km orbit which was "lost" in terms of publicly available TLEs for quite a while.
Question: Vela 1A and IMAGE "once were lost, but now are found"; are there any more "lost" satellites thought to still be in Earth orbit?
Q: Why the "scare quotes" on "lost"?
A: Because I have a hard time believing that IMAGE, a US government satellite that not only had a periapsis of only about 1000 km but was also transmitting regularly on the frequency it was supposed to be transmitting on could have gone unnoticed by all aspects of US government and military branches.