ms access training - Random pop up prompt

On our (very old) database we have a popup box prompting for a code relating to a very old query, that just pops up randomly when you're in the middle of entering data in a form. Does anyone know how to trace what's causing this and get rid of it? Clicking 'ok' gets rid of it and doesn't seem to affect the form at all but clicking 'cancel' stops it from displaying certain info on the form.