I also noticed it. It was never present on VTB before.
It got me confused and I checked the manual, Chuck's guide, and Googled it, found nothing like this that should be present on Mirage 2000's VTB.
It was not an indication @razo+r that your jammer is on. Even if you turn the jammer to standby and turn the ECM switch off, the boxed PCM still exist. Basically, once you power on the VTB when cold start, the boxed PCM will show up regardless of the status of the other systems/avionics. You can turn the radar off and the boxed PCM is still there.
Most likely a new bug.