As far back as 1974 German researchers knew that:

The proteins of molecular weights between 10,000 and 30,000 of retroviruses of a given species share antigenic determinants.

In fact they knew that the proteins of molecular weights of about 30,000 from retroviruses of many species, including monkeys, also shared antigenic determinants.

This means that the gag and pol proteins of retroviruses and antibodies directed against them are non-specific. In other words, finding an antibody to one or more of these proteins is not proof that a particular retrovirus is present. This is acknowledged by the well known HIV/AIDS expert William Blattner.

"Viral proteins, particularly the polymerase and gag proteins may be highly conserved between subtypes of viruses."

Nowadays we know that antibodies to "retroviral" proteins are widespread in human populations even when there are no retroviruses present.