I have a similar problem with rats in our shed. I seen a few rat dropping on the floor of the shed.
The thing is the shed is half full of timber and for all i know they are living in there so if i block all entrances off they will be stuck. I have a dog so poison is out of the question and not sure about those ultrasonic things. I would worry they would drive the dog nuts. I live next to a few arces of fields so mice and rats go with the territory. Had a few mice inside before but never had rats inside. My home made mouse trap does not seem to work for rats either.
Is it just peppermint oil or what ye are using and does anyone know if it will work for rats too. if it works i will put them around the yard too cause the horrible little things are always in the yard. The dog kills one every few months.