Because it's not a part of the series according to pixiv, more like a side-scene.
It could be an author's typo, but written as it is, the commentary looks like a pun on verbs "to go" and "to pass away". Please, feel free to edit the commentary if you have a better fitting translation.
Please, do not change the translation. At the time of the request it was not clear exactly what kind of legwear the artist will draw. Since the Japanese phrase do not explicitly state it either, the translation has to be ambiguous too.
Again, no need to overcomplicate the notes, since "行って来ます" doesn't have the "I'll be back" meaning expressed strongly and "I'm off!" in English conveys the same implied meaning pretty well.
Despite script being handwritten, I have no idea how one can mistake 完 for 莞. Usually it's the other way. Moreover, 莞 doesn't even have "the end" meaning.
