Hello Lubricator,
The behavior you are seeing is not unusual. Frequently the juveniles from this year’s nesting will come and investigate the boxes where they were fledged from, but they aren’t actively looking for nesting spots. They’re just “playing house” so to speak. Or, they may be investigating, looking for roosting spots for the cold winter months.
When you say you have a perch and a nesting box, you don’t mean the perch is on the front of the box, do you? Bluebird boxes should not have perches. They don’t need them, and a perch makes it too easy for other birds to harass them.
Central NY
TMB Studios