User blog comment:PurplePenguin/The Grunkle Stan Mystery/@comment-76.107.232.216-20120907014027/@comment-67.54.166.74-20130210131010

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Okay for the moment lets say I'm the one that posts the theories involving Stan training the twins, since I don't see anyone else posting them. But beside the point. to the comment above me, I see why you'd think that. But what if Mabel knows more that Dipper does? If Dipper saw that tatoo, he would get suspicous because he's weirded out by everything. But Mabel saw it. Well, if she did get to see it through all the hair...blah!

The point being that Mabel knows what Stan's tattoo means, she didn't see it, or she just didn't bother with it because she's not like that.