I would also argue that same generation nudity is awkward, my 11 and 15 yo kids would not wish to be naked in front of my 12yo, or any of their mates.I agree with this and would also say that inter generational nudity is more awkward than taboo, considering how different families work
I'd agree with both these statements, 12 is an awkward age, and I would agree that it's more awkward or discomfiting for the child rather than taboo as such. It becomes even more awkward when it's the opposite gender, as my example of a 7 and a 12 year old girl being baby sat by their grandfather who walks about naked where he knows they can see him.
If granny got naked in front of her 12 and 7 year old grand daughters it would also be perceived differently to doing it in front of her grandsons,
There is also the simple issue of adults getting naked in front of children this age, when alone with them, when the kids are not theirs and not brought up in this type of environment. Yes society frowns upon it due to the obvious connotations.
I very much doubt the woman is a peaedophile, but they exist, and as she was not regularly naked in front of her own son, and the op states she knew she could be seen, it's unlikely she forgot the kids were in her home. That would explain one mad dash, not going back and forward.
Whichever way you cut it, it's odd behaviour. The overwhelming majority of women would not wander back and forward naked in front of their grandsons of this age, when alone with them and when she spends little time with them, and is likely aware regular adult nudity is not how they were brought up.