Hi user84,
There are a surprising amount of devices out there that can create “networks” in your home. I recommend reading this article I wrote about this very thing here which will maybe help you. I discovered that both my Chromecast and Xbox were deploying networks for various reasons.
Is there a way to set up a capture on a known SSID that is not currently active?
Eye P.A. is probably a better tool for capturing on a specific channel, but inSSIDer can also do this job. To perform a capture across all channels, simply open inSSIDer and go to File > Start a Packet Capture. If you want to capture on a specific channel, you will need to dive into a network on the specific channel (using the binoculars icon) and then start a packet capture from there.
I hope this helps!