J.M.
i agree with the last post. If it was truly incontinence, then I would think that it would happen periodically through out the day, not just when she's in bed. Because that has to do with a lack of control in the bladder muscles and how can she have control the whole rest of the day and not then. My grandfather when through this after prostrate surgery.
generally speaking, when we consume something our bodies will have some sort of output about 30 minutes after the fact. you could maybe try setting a timer either after the meal or snack so that when it goes off she has to get up and try to go. She may not have to go then, but the bell going off would at least interrupt her concentration to be aware of her surroundings and body needs.