^PCG has a point. Do make sure that your food is being stolen and not that you’re losing track of how much you’re eating.
I know that food can seem like a “petty” issue to argue over. My mom often says that “khanay walay cheezay pay nahin larna chahiye.” And while generosity is a trait that a Muslim should have…MANNERS are also emphasized in Islam. Stealing (one that’s proved) is STILL stealing no matter how petty the object may be. The person who bought the food might be struggling a bit with money themselves (even if they may have more money than you do), so it’s only polite to ask rather than assume entitlement.
Assuming she can’t afford food and is having financial problems, doesn’t necessarily mean that the poster has no financial issues at all. For me, I don’t mind sharing my food. Many people have a big giving heart, but frequently taking advantage of someone in an underhanded way…is going to eventually become offensive. Of course, as you mentioned earlier, one has to be SURE that there is theft taking place.