I have attached both the Maryland Form 505 and instructions. You should file this return as under who must file it indicates "have income from sources within Maryland". It doesn't say profit, gain, etc. just income; which Maryland can construe as requiring a tax return.
Where I believe you are running into trouble, is that you have not completed page 2 of form 505 correctly. Take a look at the link and see if your return looks correct. I doubt it does based on your facts. You have most likely not completed columns 2 and 3 correctly. Once these columns are completed correctly, your Maryland return should reflect no tax due.
http://www.comp.state.md.us/forms/current_forms/505.pdf
https://www.marylandtaxes.gov/forms/19_forms/Nonresident_booklet.pdf
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