It isn't uncommon for a page to be created on Facebook, only to be abandoned. Many dealers started off with multiple pages and then consolidated to one. Others had an internet manager who has since left the dealership and they didn't even know there was an additional Facebook page out there.
Find them. Destroy them. They can do damage to your reputation.
As you can see in the image above, this is a dealership page created at the same time as the dealer's primary page. They made a page for sales and a page for service, but unfortunately the service page didn't get much attention. They had one post in 2012 and a couple of posts in 2011, and that's it...
...except for the negative comment at the top of the wall.
This isn't even the worse case scenario. It's easy for a page to be "hijacked" by spammers or competitors who post bad things. There was a relatively-infamous instance a couple of years ago where a dormant page was loaded up with porn.
Don't take the chance. Only bite off what you can chew at the dealership. There are definitely multi-page strategies that work, but they require a commitment. If you cannot make the commitment to manage and monitor more than one, don't build it. If there are dormant pages for your dealership out there, find a way to get control of them and either remove them completely, start managing them, or make them hidden from the public (just in case you want to revive them later).
Don't leave them hanging. You can end up with a bad representation of your dealership without ever knowing about it.