I'd take a trip to the local septic permitting agency and see if I could get copies of the permit and approved septic system design/as-built site plans. The inspector will probably be able to tell you if you need it or not.
It might be a vent for the leach field although that's not common where I'm from. I guess it could also be a marker for a distribution box or some other system component.
Not necessarily. Getting copies and talking to someone at the counter usually doesn't cost anything.