When some channels go off air, they're replaced by a MHEG encoded holding slide (and the channel's literally not broadcasting any more). However the presentation area for BBCs Three and Four remains live to air through the night, transmitting the holding slide until the switch to CBBC and CBeebies takes place. (The same area is, or at least certainly was, used for all four channels.)
To me, that looked like the GeeVS kit was having a bit of a meltdown and rebooted itself (or was rebooted by a friendly engineer!).
Two minutes regions...