Bilbo was probably half watching him and saw the bowl was being left idle. So as he went to take Kili and Fili their food, he was going say ‘your food is going cold’ and then saw what he was doing.

Minu got the Artbook and saw that Ori’s slingshot was his only weapon and he had notebooks and a quill.

And wanted Bilbo looking over his shoulder at him drawing and Ori getting all embarrassed.

Which made me think that it would be cute if the drawing Bilbo had of himself at the start of the movie was possibly done by Ori.