Noticed it straight away and immediately thought of Groom Lake. Those temporary flight restrictions that pop up with no NOTAM explanation are always the ones worth watching. Had the same thing happen over a site in Wiltshire a few years back - local pilots were quietly told to divert and there was absolutely nothing on the official channels about why.
What gets me is the timing. You'd think if it was routine military stuff they'd at least bother with a cover story. The fact they didn't even attempt one either means they're getting lazier or whatever it was happened fast and they needed airspace cleared in a hurry.
Anyone actually tracking the FlightRadar24 data from that window? Would be interesting to see what was and wasn't showing up. I'd do it myself but I'm a bit behind on the tools these days. Someone younger than me with better software should have a proper dig through the archived data before it disappears.