Hello, I just recently did a mileage run from DEN-MNL and back with a 75 min turn in MNL. When I arrived in MNL, there was an agent waiting at the plane with my name on it, instructing me to go contact her coworker at the top of the ramp. I did so, and they took me through the glass doors (rather than having to go through customs/immigration, etc) and I was then out in the gate area, after having gone through a metal detector, and my carry-on going through the x-ray machine (I had no checked bags, of course). They took my passport over to a computer and did something with it, but I did not get a passport stamp (even though I asked for one...haha). After this, I was asked if I wanted to visit the lounge, so I said yes and an agent escorted me up there so that I knew where to go, I stayed there for a bit (had a much needed shower), and then proceeded back to my gate to board the plane. So, I have a few questions...

1. Was the security that I went through (twice, because I had to go through it again after coming back from the lounge) just additional security for traveling back to the US (GUM)?
2. What was the passport check that they did? Was that again just part of the check to get back to the US? Or was that somehow logging me into the system so that they would have a record of me coming into the country at that time?
3. What was to stop me from just leaving the airport at that time after leaving the lounge rather than just going to my gate? I had not gone through any sort of official customs/immigration check, so I'm just confused by this stage. I had no interest in leaving the airport and abandoning my return flight, but what's to stop someone else from booking the same flights I did, going through the process I did, and then just heading out into the city?

Just curious to see if anyone has any personal experience having been through the same flight and can offer up some additional information. Thanks!