[Rounteetepehryn]

I can't add much by way of answer to this question, other than to say I'm sure I've seen/heard exactly that behaviour and explanation on one of David Attenborough's docos. I'll see if I can dig up a link on utoob or somewhere.

Notice how that interaction is between two ants, so it appears to be a defensive behavior, "hand on their gun when meeting a stranger". Nut's (and other's) observations have the ants staying in that unmoving position for many minutes.