Very well could be the tail of a 6 AWG trying to comply with 250.94. However, it should be easy to determine this once you pull the panel inside. If you can’t see other GEC’s within the enclosure other than one 6 AWG leaving then you have your answer…if you see others inside the panel then you have to look around…do you see a 4AWG going to any water pipe…?..do you see one going to a ufer…?..are you able to find any ground rods…?
If you find yourself not able to answer those questions once the panel is pulled and you have other issues in your report most certainly call it out to have it evaluated by a licensed electrical contractor…why…? simply because of liability if you really want a single reason.
IN my areas ( Richmond,VA ) you will see this all the time and not because they are trying to comply with the intrasystem bonding requirement. They are CUT because people steal copper all the time. In a single day with a concentrated area a THIEF can earn quite a little take on stealing this…Why hold a sign at the street corner beggin for a dollar when you can steal copper in 1/3rd the time.