At the end of the series, when Durandal/Thoth thanks the Security Officer and lets him go.
Durandal/Thoth: To you, we are deeply grateful, and release what little hold we might, as Durandal, have had on your soul. Go.
Just about any time Durandal opens up to the Security Officer in M2 and Infinity, however briefly.
Examples from the later fan scenarios:
In a reference to the ending of Marathon Infinity, the ending screen in Rubicon has someone (presumably Durandal) accept that the Security Officer single-handedly did everything they have asked of him, and if it weren't for the SO, everything would've been for naught.
"For you know as well as I that you cannot be Destiny without being a Hero."
