If anyone else comes across this I found another reason: in an app in dev I only have references to the icons; I did some disk cleanup, moved them, then got this message the next time I tried to Build/Run.

Interesting thread. My apps have been in the store for years, haven't revisited this. I remember you could create a small number (25 or 50?) free codes for tech press to review your app. Long gone, correct? Unfortunate if true. Word of Mouth is the main approach for small indy's to get their app some exposure.