As predicted the eagerly-awaited “secret song” as tweeted by Bill was the Serbian folk song Ajda Jano.
Blic features an interview with Biljana Pantic, the woman who joined Patton onstage to dance during the song.
She revealed that she was subsequently invited back stage, that the band were very polite and grateful and that she received a drum stick from Puffy. [Any translation from Serbian readers would be gratefully appreciated].
Cheers to Lazar and Male Practice for the translations below the line. Much appreciated. Blic also has an English version so you can read an overall festival review and the interview with Biljana.

Video:
Out of sync pro video below

Second take:
Second take fan video

Photos:
Mondo.rs
Blic

Set list:
Reunited
From Out of Nowhere
Land of Sunshine
Caffeine
Evidence
Surprise! You’re Dead!
Be Aggressive
Last Cup of Sorrow
Cuckoo for Caca
Easy
Ashes to Ashes
Ben
Midlife Crisis
The Gentle Art of Making Enemies
King for a Day
Ajde Jano
Epic
Just a Man

Ajde Jano (take 2)
Chariots of Fire/ Stripsearch
We Care a Lot