Posting to follow.
I'm wondering about this a little bit myself. We elected the full FSA amount during open enrollment for 2016, and then my wife spent 10 months of 2016 without employment.
In our case, she was actually seeking employment, and has an email folder full of "Thank you for applying" type emails. I don't know what kind of substantiation we're looking at when we go to file. I'm hoping it's just a checkbox of "yes, I was seeking employment" or something.