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This answer to What “missing satellite” was part of the story of CNN's debut? quotes the 1981 UPI Article The RCA Satcom III-R commercial communications satellite, designed to... as follows:

The original Satcom III satellite was launched successfully in early December, 1979, but it disappeared after a spacecraft motor was fired three days later in an attempt to place it in a stationary orbit 22,300 miles above the equator.

Despite sophisticated tracking systems, the satellite was never located and its fate remains unknown.

and links to the New York Times article RCA Loses Contact with New Satellite which says:

RCA publicity materials said the boxlike satellite was about five feet by four feet by four feet and weighed about a ton.

It's hard to understand how such a large metal object could remain lost forever "Despite sophisticated tracking systems..." Assuming there were at least large pieces of it intact it should still be visible both in satellite tracking cameras if not by space junk-tracking radar. Even in GEO satellites can be seen by amateur telescopes.

Question: How could this satellite be actually lost forever? Are there at least semi-authoritative theories? Or perhaps was it found again much later? (these are 40 year old articles)

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Or perhaps was it found again much later?

It appears to have been found. According to Wikipedia, Satcom 3 is COSPAR ID 1979-101A; Celestrak has a current TLE for it.

SATCOM 3                
1 11635U 79101A   20165.54218687 -.00000091  00000-0  00000-0 0  9990
2 11635   9.1831 306.7881 4819048  43.5856  92.1293  1.82528373183904

That's an eccentricity of 0.482 and almost 2 orbits a day, so well out of geosynchronous orbit. N2YO says 35,541 km x 8,282 km on June 13 2020.

TLEs for the satellite begin reappearing with November 18 1982 with a eccentricity of 0.479 (significantly different from the 0.723 of its 1979 track), but I couldn't find any information in a few minutes of Googling on how it was rediscovered.

A debris field associated with Satcom 3 was identified in 2006, but the major remnant of Satcom 3 has a large radar cross section (~6 m^2) according to N2YO, so whatever took it out of commission didn't break up the bulk of the vehicle.

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    $\begingroup$ Excellent! A check of space-track.org shows the first TLE in the 1980's is on 18-Nov-1982. $\endgroup$ – uhoh Jun 14 at 3:31
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    $\begingroup$ Thanks for the link to the 'satellite fragmentation' document. My copy is old and it's nice to get a new one. $\endgroup$ – Organic Marble Jun 14 at 16:48

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