This StoryMap was created and published over a year ago, and has had much use with no previous sign-in issues. In the last few weeks (perhaps longer?), users are now prompted to sign in somewhere in the first chapter. If you hit cancel enough times, all the maps load and we can't identify any layers that are not present (although there are a lot and this is a long StoryMap).
https://storymaps.arcgis.com/stories/07d8fa3cae6843efb476a67d28c81e99
Note: We now see the blue key symbol (OAuth 2.0: A registered OAuth 2.0 application. May use premium or subscription content.) This was NOT there when the StoryMap was published - and we have not modified anything in over a year.
We presume this is a premium layer issue, where a previously non-premium layer got changed to premium, but have been unable to track down the offending source. There are a lot of maps and scenes with a lot of layers in each.
a) Is there an efficient way to see which layer and/or map is causing the error?
b) Within a map/scene/folder etc is there a way to filter out or view which layers are premium without having to open every layer within every map?
c) Do non-premium layers sometimes change to premium unannounced?

Note: I can't identify any layers via the browsers Developers Inspector tool that are showing a 401 or 403 error.
@Amanda_Huber
Thanks!
Katie