I was wondering about Okiya's comb on that page. There was no real reason to show him pulling it out, after all; and the way he pulled it out made me think of it as being some secret weapon.
I can't help but think that the exposed neck was a bit of a red herring. It's obvious that something's there; Ai tried to check under his neck specifically for something. I thought of this the other night and thought that maybe it was meant to distract from something else in the scene. On one page, Sonoko was commenting on how he hides his neck. When it appeared the very next page, we would naturally focus instantly on his exposed neck to look for any marks. The first time I saw it, I went back and forth between both pages to look at the multiple angles shown of his neck before continuing the story. Looking back, it was like the misdirection used in several of the cases, distracting us from something else. Of course, it may have been the fact he was brushing his teeth right-handed.
However, there's another possibility that just occurred to me: if Yukiko was there at the time, she could have used her make-up skills to cover any marks. The woman can create latex masks and is one of the best disguise artists in the series. For her, creating foundation that perfectly matched Subaru's skin tone to cover a mark on his neck would probably take only a matter of minutes. I looked back to the two tooth-brushing introductions, and in the first one (the Paper Airplane case), part of a door could be seen in the third panel of Chapter 638, Page 9:
Now, it isn't the bathroom door, as the angle of it is different in the previous when Ran and Sonoko are looking inside:
I also checked out the corresponding anime episode (510, around the 17 minute mark) and the door is shown much clearer there. It even shows Sonoko entering the bathroom from a totally different door. Most likely, it's a closet. If Yukiko were there, she could have hidden in the closet after applying make-up to Okiya's neck. Just to be safe, I compared the bathroom from that chapter to the latest one, and they seem to be the same:
The only reason I thought about the closet thing is because they'd want to minimize the risk of the foundation being smudged by his collar or something, totally negating the original point of it. However, even if Yukiko didn't hide in there, the possibility is still very real. Also, while looking back for these, I reread the whole hiding-a-woman conversation and realized that Conan was probably in on his mother's presence even then. Chances are pretty good you guys already figured this out and I just misread the conversations about it, though, since you've discussed this before.