My working life started at Waddo in the 60s, and like you, my memory is that at no time was it ever mentioned that it was an exercise when the hooter went.
When a certain Vulcan Sqn Cdr complained at the Debrief that it had taken him over an hour to determine whether it was an exercise or not, the Stn Cdr very reasonably replied "And what difference should that make, Chris?"
and a Taceval Pt 1 that started not at 00silly but 1220
OMG, we had one of those too. It was utter mayhem, with people trying to get vehicles off the station and then get back in again on foot, and nobody in the right place anywhere! IIRC it was well over an hour before the 'normal rhythm' was established!