I am not sure if this qualifies as "hidden" for you, but you can add "notes" to "nodes" the following way:
Right-click on the node you want to annotate. This will bring a context-menu. Choose "Add Notes" --> Type your note in the "Node Notes" field which comes up.
The note is now a mouse-over note. A small pen icon in the left part of the node indicates that the node possesses a note. When you mouse-over the pen the note will come up. This even works in the html-version of the map.
This message was edited 1 time. Last update was at 08/13/2014 05:23:20