What does an MIA agent mean? A user upgraded to a new laptop and has shelved the old one, it's gone MIA in fleet but it will be used some times, will it reappear in fleet when it comes back online or does MIA write it off and it needs to re-enrol? (I scrub secret after enrolment)

It means the agent hasn't checked in to fleet for 30 days. It will not need to reenroll as long as it's not deleted from fleet (and the osquery database is still available on the host).

Ok that's alright then thanks, It can live as MIA.