There is a strong septic smell in the laundry room and the master bath which are right next to each other. It was raining that day. The smell was very strong. We ran the washer and the smell was not inside the machine. It is a ventilation problem I think or the plumbing. The smell was very strong. The house is 2012 house so septic tank should not need flushing. We opened all windows and ventilated house but smell came back. Experience home inspector on site and could not find problem.

Do you have a sink that's almost never used, or a floor drain in that room the trap might have gone dry in?

Your vent stack should pull air IN to vent draining but if you have a blockage it can push sewer gas out which can be drawn into the home. Do all your drains work well, no gurling or bubbling?