Happen to discover (4,2)
I believe the answer is:
turn up
I believe this is a double definition.
'happen' is the first definition.
(thesaurus)
'discover' is the second definition.
(thesaurus)
'to' acts as a link.
(Other definitions for turn up that I've seen before include "make louder" , "Appear by chance - increase volume" , "surprise" , "Increase strength" , "part of leg" .)