2013 Ford Focus Brakes Issue?

So I started my car yesterday and realized that the brake pedal didn t depress like it usually does after I start my car.

I made my way to the local mechanic (using the parking brake and engine braking), and when he opened the hood while the car was still running, there was a constant stream of air coming out of a tube.

When he put plugged it with his finger, I was able to brake normally. Apparently whatever "plug" was supposed to be there has somehow gotten loose.

Needless to say, he found a random plug in his toolbox and sealed it up.

Questions:
What could have caused the plug to come out?
What the heck was the issue?
Is it common for this type of thing to happen?

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