I think it was because the person felt it was probably just a good time in general. And whoever it was put a card in the door to the motorcycle court, so that it wouldn't engage and lock. You could see the card falling when Daryl opened the door. So he/she knew that the card probably wouldn't stay there very long, so he had to go now.

 

I suspect it was primarily just because Negan and a good proportion of his men were away and otherwise engaged.

I'm more interested in who gave him the note and who put the card in the lock, favourites have got to Sherry or Dwight but maybe it's someone else. Maybe working with the Kingdom or Hill top.

Something else that concerns me is what sanctions Negan will take when he finds out Daryl has gone? Daryl must surely know that Negan will probably take action against Rick and the ASZ, it's his style.

 

I also think Sherry left the note because she wants to help Daryl out of the predicament. She knew Negan was away and had some of the men with him. She most likely knew where Dwight was at the time also.