I've done this many times on UTCS. You know the ledge above the main exit? Build a ret there, make sure it has power, go aliens, preferably maruader-goon, stand [not jump etc.] in a certain spot where it winds up. It should shoot you but miss. I've known this for like 2 years.
Didn't watch the demo but I'm fairly certain you are inside the turret's deadzone. The turret can see you but it cannot point the barrel at you due to the mechanical limitation.
I think its an error in the source code. In 1.1 it was fine, but in gpp the mechanical limitation is now 30 degrees down rather than 45, but it still tries to shoot at 45. About 'detecting', it always detects. You notice this by running around underneath the mgturret, it should turn but not fire.