I have seen this, or something very similar on various houses in the city I live in. It is rare enough to be remarkable, but common enough to make me think it is a "thing". Any ideas?
That is a balloon bird repellent device. It is designed to mimic the appearance of an owl to scare away smaller birds and prevent them from building nests in the eaves, or to prevent them from flying into windows. See https://www.amazon.com/Balloon-Bird-Repellent-Effective-Solution/dp/B01DAFTNEU