If a NOTAM had been issued it would have appeared.
Although fixed wing tend to not stray below 500 feet helicopters seem to have more freedom - as well as military popping up well away from recognised low flying areas there are also Air Ambulance, search and rescue and Coastguard.
I have been in a Coastie Sea King on my way to the decompression chamber at Haslar, having made an emergency ascent from depth. That flight never went above 100 feet.
So, as well as military pilotsbeing allowed anywhere, there are legitimate flights which are made in an emergency for which no prior notice is possible