Thank you for your question. Your situation is certainly nerve racking as your employer is leaving you in the dark as to what the interview will be concerning.
However, at this point, your employer has not done anything which triggers any legal rights. Michigan is an "at will
" employment state which means that the employer has a right to terminate at any point for any legal reason unless you have a written contract which specifies the reasons for which you could be terminated. I bring this point up because it also grants your employer the right to withhold information from you at its choosing, including the subject of any sort of investigation.
There is no telling what the interview is concerning at this point and unless you are aware of something that is in violation of company policies or some illegal activity, then you should simply be prepared to answer questions.
Of course, this could also be the sign of a lay off as well. Hopefully that is not the case.
If it is regarding some kind of investigation, be prepared to be asked to sign either a new company policy document. If they accuse you of doing anything illegal, do not sign any documents.
Please let me know if you have further questions.