Use of :@ on mode line

[Kevin Buchs]

I'm running GNU Emacs 25.1.1 (x86_64-apple-darwin15.6.0, NS appkit-1404.47
Version 10.11.6 (Build 15G1004)) graphically and since I started running
it, I'm seeing an extra character on the mode line. Where there was just a
colon between CS and CH, there is now :@. What does that mean? It isn't
documented in the Info manual:  (info "(emacs) Mode Line") nor do I see it
in the What's changed section of the Info manual.

 Example mode lines:
  - Reading an Info page in info-mode:   U:@%%-
  - Buffer created and changed: U:@**-
  - File opened and unchanged: U:@---

Thanks,
Kevin Buchs