While this feature was not introduced in the first build of iOS 8, the icons show that the feature was being actively worked on during the development of the mobile OS. iOS 8 was rumored to launch with many new Mapping features, including transit directions and improved data, but none of enhancements made it to the first build.
We assume Apple would use GPS along with the M7 co-processor to help track movements from and to your vehicle to help you locate it quickly.
Yesterday, a repot revealed just why iOS 8 never saw that Maps upgrade it needs. Thanks to developers leaving, poor management, and internal politics, we may not see an maps upgrade until later.
In addition to this hidden icon, a transit icon surfaced yesterday in a WWDC session video that hinted at the possibility of Transit Directions coming to iOS 8.
It is unclear if these features will make the final iOS 8 build, as Apple does sometimes save a couple features until the GM release.