Septic tank odors are coming from the air conditioning vents. Air condition systems do not create septic odors however; they are able to recalculate septic odors making it appear that the odors are worse then they really are.
This is usually caused when a sewage drain line or a septic drain field is exposeing leaking toxic effluent into the environment. The air conditioning ventilation system will on occasion pick up the septic odors sending them through the system.
Anytime septic smells are obvious, it is important to contact a plumber immediately. Turn on fans open windows and doors in an attempt to allow fresh air to inter the home displacing septic gasses. Evacuate family and pets immediately.