Yeah at first I also thought it was a Casper thing. But I would remove the JAMF framework and also remove the devices from the JSS, and would still get the prompt.
I have a test box with a small SSD (i.e. fast encryption) I can test this with. I've been meaning to confirm my findings anyway. This drove me nuts for quite some time.
OK so I did my little test and threw in @themonger13's suggestion into the mix.
Log in to invisible management account. No prompt.
It seems @themonger13's suggestion works.
Press on to confirm whether or not El Capitan 10.11.4 corrects the problem:
Log in to second local admin account and delete /Library/Preferences/com.apple.fdesetup.plist.
@themonger13 wins the beers.