That is when you get notified of someone else's AirTag that might be traveling with you
it does not show you the route of the Airtag really, it shows you where both you and the AirTag have been so you can see how long it has been traveling with you
See:
What to do if you get an alert that an AirTag, Find My network accessory, or set of AirPods is with you - Apple Support
Find My displays a map of where the AirTag, Find My network accessory, or set of AirPods has been observed with you. The red dots show where the unknown item was detected near your iPhone, iPad, or iPod touch. The dashed lines connecting the red dots help indicate the sequence where the item was detected with you. The red dots do not indicate when the item's owner is viewing the item's location.