They are "breadcrumbs" and will populate the screen anytime you are in a location that is not a recognized road. Their purpose is to be able to have the system follow it's own trail back to where you came from if you needed to go back - either go back in the future or if you needed to immediately back track.
These areas inlcude any new roads that are not in the database yet as well as any shopping mall / shoping plaza parking lots. The closer you zoom, the more you'll see. I keep mine at 1/4 mile and can see many of them around the places that I go to all the time.
You can turn the "breadcrumb" tracking feature on and off (it's found somewhere in the setup menu), but if you turn it off, it will not erase any previously created breadcrumbs.
Hope this helps.