I have just installed Ubuntu 15.10, after having been with 16.04 LTS for some time to try it out. While I was using 16.04, I tried to install Nvidia Drivers and got told that I needed to disable SecureBoot for that, so I provided a password and on the next reboot some software asked for it. However, my BIOS settings still showed SecureBoot to be enabled, and I could still see how if I tried to boot from anything that wasn't signed, it would refuse to do so.
However, when I reinstalled Ubuntu 15.10 (completely deleting everything related to 16.04, including grub in the EFI partition), every time I boot I get a
Booting in insecure mode message.
I have checked everything, and both: Windows and the BIOS setting report SecureBoot to be enabled