He talks about this in his book “No dream is too high”. They didn’t know what it was and knowing about UFO enthusiasts would be listening, decided not to tell Mission Control until they got back (if I remember right).

There were two main options. It could have been the final stage. But they figured out that the final stage was too far away for them to see it at that point.

So probably it was one of the four panels that got released during a maneuver they had just done. They would normally just fly away into space and get lost. But one of them probably somehow got into a trajectory similar enough to their spacecraft for them to see it. I can’t find it right now, but was reading the book yesterday. If I can find it I’ll do a quote here about it with more details.