Is there any legal option for a property owner to inspect a unit without notice? In other words... Can a property manager do a unit inspection (spot check) without a notice being given to the current resident if there is no actual emergency?

Probably not, unless the lease or a specific statute in whichever state you are located says otherwise.

But why would the landlord want to do an inspection without notice, in the absence of an emergency?

It's not a prison, it's a rental unit.