Let's put aside the fact that I can relate to this story of love and infidelity far too easily. Munro is superb at getting inside the emotion of relationships by viewing them through a character that may or may not be honest with him/herself about it. After all, perspective and perception play the biggest roles in such things.
This, of course, is what Prue tells herself to justify her taking the cufflink. We suspect it is not quite as flippant a gesture as is suggested. But it tells us something about her that contrasts with her more cheerful and accepting outward appearance on relationships, and this one in particular. Though Gordon has indicated that some day he thinks he wants to marry her–the ex-wife being out of the picture by now–he also admits that he may be in love with the woman who has angrily interrupted their private dinner, and needs to get over her first. (Whatta guy!) They do not seem to reach a happy ending together, and Munro brings us back to this little detail of the stolen cufflink.
Munro cleverly lists all the things the cufflink does not symbolize or her reason for taking it. Yet, we've already gotten the idea that Prue isn't exactly what the world–or even she–sees her as being. We may think she's a fool for falling for a sap like Gordon, but she doesn't appear to feel that way and we cheer her on. The cufflink may be the one part of him that remains permanent for her. That is there between the visits. It may be something she takes for herself from the life he does not allow her to share. It may be a grounding, a nest. Or, more simply, and I suspect this from experience, a small way to inconvenience him (where does one go with a single cufflink?) that somehow balances out the inconvenience she so cheerfully accepts as part of this on again, off again relationship that he manages to control.
We may not know the reasoning behind Prue's actions, but we can't help but understand that she does what she does for her own peace of mind.