I think the important thing is to NOT file as a NON-resident (or as a partial year resident), and not necessarily to file a resident return if you are not required to file at all.
However, as a resident, you have to report all income from all sources, so if you have grants or interest income you would report them. Even of you don't have any income whatsoever and you are not required to file, you can file and have a copy if need be.
I would like to hear about situations other than being a student. I understand people who do not study get re-entry permits approved, and I'd like to know what they stated as reasons. I guess another reason would be to care for somebody who needs care, but that is not my case either. Can I state the reason as "personal" or will that be too vague.
Moreover, I need to work full time in another country, and I was not sent by a US company or government. Would that be permitted?