For a while now, Grimm-watchers have been copping on to how Sergeant Wu may know more than he's letting on about how Wesen seem to run rampant all over Portland. But so far, there hasn't been much to give away just how much Wu knew.
That may have changed in the latest episode "Red Menace," where a human assassin out to get a Wesen assassin gets a fatal dose of radiation poisoning by that same Wesen (named Mishkin). There's a scene when Nick, Hank and Wu have to take exfoliating showers to decontaminate themselves after being with the dosed killer, and it's one of those scenes that can be taken a number of ways, one of which is a nod to how much Wu knew.
They're nattering on in the showers about how the killer got dosed, and it kind of feels like two parents talking in code about a birthday present in front of their kid Sergeant Wu.
Nick says the killer must have been exposed when he went into the store room, and Wu says in a sort of drawn-out, knowing way "...And by exposed you mean..." And Hank responds directly to Wu: "Mishkin!"
That scene you can take a few ways:
Way of the Scene #1: They're just re-telling the part of the plot that gets re-told every week. Grimm is the Jimmy Two Times of genre television.
Way of the Scene #2: Wu is making an "exposure" joke, and his double entendre irritates Hank, who sets Wu back on track by snapping "Mishkin!"
Way of the Scene #3: Wu is on to Nick and Hank's parent code; he already knows about Wesen, and he knows that they know about Wesen, but they don't know that he knows that they know. Hank may be suggesting that Mishkin dosed the killer Alexander Litvinenko-style (with something like polonium), but for Wu (and us, since Wu is a great stand-in for the audience), he already groks that Mishkin is activating his Wonder Wesen Powers to slowly melt the killer at the cellular level. That's why he smirks after Hank says "Mishkin."