Oh cool. Are we going snipe hunting!!!!!!!?
I'll give the OP the benefit of the doubt. I'll bet the "unexperienced source" said something like, "when I/my Mum/my Grandad went to university in the 1970s, young folks used to go camp out at the airport and..." (The OP is from the UK, so Mum would have gone to university. In the US,Mom would have gone to college.)

OK, yeah, back inthe70s a friend "hitchiked" across Alaska by hanging out in regional or small airports and asked pilots readying their small planes for flight if he could pitch in for gas and come along. It worked and he flew with private pilots from Anchorage to Ketchikan hopping from small airport to small airport.
Other than that, ready your catch-bag for the snipes.

I meant how would someone get past security now without a boarding pass.
I remember the security "light" days myself. Though they are funniest when seen in an old movie.
Oh, I got ya. Yes that's a good point. Can't really get to a gate to hang out waiting for a last minute seat anyway.
I guess you could hang out in the terminal, but as stated there's no reason to.
Also the practise of overbooking is far more common in the USA than anywhere else. Where you can benefit, is by being flexible and taking up the voucher offers that get offered to unload passengers.
I was offered a $400 voucher to offload on a flight from Dallas to Guatemala. Which would have been fine, but as an Australian a $400 voucher for an American airline isn't much use to me. In hindsight I guess I could have ebayed it for $300 :-)
@SusieGirl17
Many civilised parts opf the world still do not enforce draconian security.
In Australian domestic airports one does not need a boarding pass to go to the gate, and many countries are the same. International travel is a different ball game, but domestic airports in many places anyone can go in regardless of holding a boarding pass. In fact where I live I will often go to the domestic airport to go to the bar there or the food court as its sometimes close to where I am working. As long as you go through the security screen, anyone can go in.