Erwin is not, and never has been, a stupid man. Nor is he someone who can't read another person, usually so he can use that information to his own advantage. With Levi, though, there's an odd amount of blindspot, particularly when it comes to certain topics and activities. They've fooled around since that first time, each time successfully and to Erwin's great enjoyment, but Levi has never been either vocal or insistent on his own wants or needs. Erwin always makes sure to check in on him, and if Levi says he's enjoying himself, Erwin takes him at his word. He figures Levi, of all people, would know exactly what to say or do to get Erwin off him if he wasn't having fun or suddenly changed his mind about whatever they were doing. But taking his own initiative, or actively pursuing his own pleasure? No. That doesn't seem to be anything on Levi's radar, and so Erwin has stopped expecting it.

To put it another way, he would have noticed on anyone else that his presence and touch was having such an effect. But considering that it never has on Levi in the past, Erwin stopped expecting and looking for it, and is thus unaware of Levi getting wound up.

He starts to get an inkling, though, when Levi doesn't answer him right away. Erwin was half-expecting a hand to his face or a shit joke, not that odd silence and Levi staring at him like he's taken a header off a horse and rung his bell. Interesting...

"All right." Erwin leans back enough to get his hands between them, and makes a silly, over-exaggerated little show of undoing Levi's belt. He can take off a belt with one hand, but it's much easier with two, and it's also more fun. If this doesn't get Levi complaining, nothing will, and then the only logical conclusion left will be that Levi is into this, and that knowledge? That knowledge will get Erwin cranked from two to a thousand in a fraction of a second, and everything about this will change.

If, however, Levi bitches about Erwin showing off, then he'll get a leg massage and nothing more.