On a Motorola Type II trunk system, scanners don't track talkgroups the same way the actual trunk system radios do. In each electronic message sent to the radio, the last digit of the talkgroup (simplistically speaking) identifies certain properties or abilities of the talkgroup or the current transmission. Early trunktracking scanners did not recognize this as a status function and instead treated every value as a different talkgroup. Modern scanners have the ability to ignore the status bit. Realistically, the status bit should be ignored altogether permanently for the end-user. It only serves to add confusion like 69stiles experienced.
Type II "status bit" values can be viewed on
this page of the RadioReference wiki.
If you are picking up 8704 on 8707, that means the talkgroup is patched to another talkgroup. Running a program like Unitrunker would tell you which TG that is (if you can't be bothered to listen and find out which TG is broadcasting the same audio), but that's an entirely different discussion.
As for changing the status of a car, that's done with a different function in the trunk system. It's transmitted on the control channel but is not broadcast on a talkgroup - it's just data sent from the radio to the trunk controller.
