Keep in mind three things need to occur for someone to activate and travel through the Royal Road.
1. That person must be a Key.
2. That person must be at the Significator location (the location where the Royal Road touches in that arcana).
3. That person must be focusing on their destination (which would be the Fool if the person is any arcana other than the Fool).
As you can imagine, random travel by people who are Keys but do not know it to the Fool is rare but it does happen. In Mandrake's case, it seems someone painted the jester graffiti on the wall at a Significator location to help guide Keys who might not know who they are. As a Key, Mandrake can sense the spot is special as a sort of vibe running down her spine.
See everyone Thursday with a big announcement!