I would think that she wouldn’t have been away from her baby for very long, but ? Could any of the following be true? [Note: I know very little about the 1931 census so the following may not apply.]
Was she away during the census taking; perhaps briefly taking care of a relative.
Was she ill and admitted to a hospital (although I’ve not yet found her in 1931).
Could her husband and children have just been visiting his parents for a short stay; perhaps she was too ill to travel away from home.
Was she ill and her husband and children left their home to avoid catching her illness.
Did she briefly return to any family members for reasons unknown to you.
Did any relatives live in neighbouring provinces? Have you searched elsewhere for her? But, perhaps you are correct; maybe she was accidentally omitted. 
Hello Lisa,
Based on family stories, I know that Phoebe died very young (age 47) and was ill for quite a few years before hand (probably starting in mid to late 1940's?). Because of this I did consider the possibility of her being hospitalized but couldnt find any hospitalized person that matched her description.
Based on family stories, Phoebe does not seem like the type who would choose to be away from her young children. With that said, I know that William's parents were not very fond of Phoebe because she was not Catholic by birth. I believe that she converted around this time she got married, however it is implied in stories that that was only done as a formality. I dont think this would have been enough to cause her to live elsewhere but if it was, I am at a loss for where she could have went. I have found her parents in the census and she had no other siblings she could have lived with.
