so....when?

I have a question. When is it appropriate, if ever, to admit that you have been listening in on a phone conversation to which you have not been invited?

Right now I am listening to my coworker rattle off to her husband on the other end of the line, a list of groceries she needs him to pick up. Well, one of the things she listed was a bag of shredded cheddar cheese. And she said "and I need two cups of that, so make sure you pick up two bags." The thing is, a normal bag of shredded cheese (any flavor) comes standard in a two cup sized bag. So, really she would only need to buy one. Although, sometimes, they come in a three cup sized bag, they never really come in a 1 cup sized bag. Anyway, so I wanted to lean over and tell her that he really only needed to pick up one bag, not two. But then the jig would be up; I would be exposed for the listening tom (?) I am....

I am kind of pathetic.