Two Homes with For Sale Signs are not Multiple -Listed at Sabino Mountain

I scratch my head since there are two homes with For Sale Signs–one on North Quail Canyon and another one on N Boulder Canyon >(at the end) that are not listed in Tucson Multiple Listing Service. Both signs state they are represented by Coldwell Banker. It would make sense to me if they are serious about selling the homes they would be listed in Tucson Multiple Listing Service so the agents would know they are for sale and they then show them.