The odor at that point isn't THAT strong. Det. Arndt, being familiar with this kind of thing, recognized what at that point is more like a sickly- sweet odor. More like a person who hasn't bathed in a week or so. NOT like what the full-wet-decomposition odor would be. Putrefaction is a horrendous odor. Neighbors or passers-by would smell it. But JB was nowhere near that stage of decomposition.
A lot of what Arndt smelled was the stale, dried urine too. That would also not be too strong. Kind of like a cloth diaper that had dried with urine in it. FW probably spent seconds, not minutes, looking in the WC. And he was undoubtedly stressed as anyone would be in that situation. The odor wasn't strong enough to hit him in the face. It isn't unusual that he didn't notice it.
Ahhh, I get it now. This really helped me to sort out some things in my mind. Poor FW, Ive been at him hard and he didnt even do it (LOL) Yet I strongly believe that he knows who did. I bet it haunts him.
There are things that we don't want to happen but have to accept, things we don't want to know but have to learn, and people we can't live without but have to let go.