So she's obviously someone of some importance that snuck out, otherwise she wouldn't need to hide herself in that manner. Which leads to speculation that Ibis is another sister, different from the sick one she's questing for; or that Ibis and her sick sister are one and the same, and he's visiting a known stand-in, to keep the fact that Ibis is sick hidden.
There's also the worst-case scenario, where she somehow pulled all that shit out of her ass and has a completely
different reason for getting the moongrass (or just getting into Yggdrasil altogether), or some other objective entirely that she couldn't fess up before Hyce, which can further heighten his distrust to other people than it already was.
Not saying it will happen, there's a
veeeeeerry small chance of that happening given what we've seen so far, but I'm playing devil's advocate here.