URL of page "host"

Hi,
When opening a node from the fullcalendar in colorbox, I would like to know if the node was opened from fullcalendar or in the ordinary way. Let me explain:
I have created an event node. In the node, I say "If you opened this event from the Calendar, click here to go to the event (for instance to register for the event)".
I would like to only show this sentence if the event was in fact opened from the calendar. How can I test for that? The URL reported (eg. by $_SERVER['REQUEST_URI']) is the event path, regardless where the event was opened.
Is there any way to work around that - for instance to discover if the event is currently shown in a colorbox or not?