Let me explain what I mean a bit better, I'm not saying that someone has got rid of the real Frau and they are pretending to be him, the Frau we saw with Teito by the fountain was the real Frau, but the Frau that Teito saw on the building was a fake, this was done to make Teito think it was Frau who killed the criminal and to maybe try and turn him against Frau, we know Ayanami's people are in there so it could be possible, I was thinking something like that.