Cheap flights by waiting at the airport?

Nope. Wrong. Urban legend.

Most flights today are deliberately overbooked so that there are no empty seats.

I have also watched the last seat on a plane go for about 10X what the advance purchase seats cost, just days before departure.

You'll be living in the airport for weeks. You'll also have to make a career out of checking fares. One-way flights are often more expensive than round trips.

Or, you might find out there is a better last-minute deal, but it is on a flight from a different airport 300 miles away.
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Kahua is correct; however, just so you know the source of the legend.

It did used to be that way. Back in the day, before 21 and 14 day advance purchase rules, there was such a thing known as flying stand-by. Stand-by has pretty much gone the way of the dinosaur, but once upon a time you could get the very last unclaimed seat(s) on an airplane for big discounts.

That was a while ago though.

How would someone clear security without a boarding pass?

To expand. Many, many years ago, airlines did indeed discount last minute tickets. But as computer analysis got better, they figured out a couple of things.

First, airlines could better decide on flight frequency and airplane seat configuration, in order to minimize empty seat.

Second, they figured out that last minute fliers were mostly people who had to fly with little notice and would pay whatever it took. Most of these are business travelers whose employers are paying for the flight. So the airlines eliminated stand-by fares and began charging last-minute fliers full fare.

The only cheap stand-by fares left are for airline employees and their families.

SusieGirl, Back then anyone could walk to a gate.

This was before the need to have a boarding pass in order to get to the gate areas.

Lake: Ah, I remember those days. There also used to be military and student standby fares.

That was back in the "olden days" of the 1960's - early 70's. By 1977 those standby opportunities were already a thing of the past.

It's not like all flights are run full to the brim. Most still have at least a few empty seats--at least, most flights I've been on, which is plenty. But no, you don't get the cheap fares by sitting around in an airport: you get the cheap fares by staying at home in front of your computer and obsessing over them.

Wasn't so long ago I used to fly on round trip tickets and sell the unused half of my ticket to strangers I found by running want-ads in newspapers. I also took driveaway cars cross-country. And minimum wage was $1.65 an hour, AFAIR.

Mark

Sounds like someone is sending you on a snipe hunt.