I think you may be right about hogs not always knowing another one is approaching – especially if the wind is in the wrong direction. Not sure whether you may be over-interpreting the ‘defensive’ position, though – they may have just stopped in their tracks to listen/smell. I think they sometimes hope they are invisible, even when aren’t. One little chap here has been seen to approach from one side get rolled up and then sneak off and make a big circuit to try to approach from the other side – could be to do with the wind direction – don’t know. But if the boss hog is around, he usually notices and the poor chap gets rolled up again.
Some of the hogs certainly seem to be very clever at avoiding being caught on camera. Some special infra red invisibility cloak maybe.
It’s nice to hear you had 4 hogs at once, even if half of them were rolled up!