This morning, as I was walking to university -- so around 8.55am -- I heard a WWII 'Moaning Minnie' air-raid siren being sounded. It seemed to be somewhere about a mile away (you could hear it above traffic noise, but it was clearly attenuated).
I was immediately fearful that this heralded a nuclear attack.
The only thing that prevented me from starting to panic was a florist setting out her stall nearby, who carried on calmly as though nothing out of the ordinary was occurring. I assumed therefore that she was familiar with the noise, so it couldn't be an emergency. (Now I realise that this was probably an unsafe assumption, she could have been deaf for all I know).
For the 10-15 seconds or so in which I didn't know what was going on, I was right back at school with the nuclear paranoia I thought I had grown out of. I was even looking for open doorways so I could duck inside. Scary how this stuff sticks with you.
I have absolutely no idea why an air-raid siren was being sounded during rush hour in SW London.
We had several End Of The World scares when I was at middle school (1980-1984), all of which (from recollection) scheduled the appointed hour at 10am. On one occasion -- the last I suspect -- the headmaster got wind of it and prolonged that morning's assembly until 10am was safely past. This didn't reassure the doom-mongers, and news promptly spread that The End Of The World had been postponed until 10pm instead. Oddly, I don't recall anyone turning up at school the next day seeming relieved. We just carried on as though nothing untoward had happened.
No weird tests that I can recall, though.