DJM-850 - Effect Channel Select Sends Audio to Headphones

Hi,

So, I lent my DJM-850 to a friend over the weekend, and now regardless of any of the cue buttons being lit, audio gets sent to my headphones depending on which channel the effect channel selector is set to.

My friend claims "nothing has happened" so not really any help there.

Could this be a PNLB or a Main Assy error?

Thanks.