Okay, looks like I tracked it down (it just wasn't documented anywhere).
For anyone else looking for this, the value can be found by calling event_list_attendees
and there is a value called affiliate
stored for each attendee.
E.g.
Object {attendees: Array[1]}
+ attendees: Array[1]
+ 0: Object
+ attendee: Object
+ affiliate: "TRACK" <--- The widget referrer
amount_paid: "0.00"
barcode: "123456789012345678901"
created: "2013-09-15 09:32:42"
currency: "USD"