That was my first thought too. But I wonder if he was still sitting upright. In which case, the cleaners would have bypassed the stall.
That's what I'm thinking. Most people are going to avoid even talking to somebody sitting in one of the stalls. I can see somebody being left alone in there, and a foul smell wouldn't necessarily be suspicious. Though... you'd think after a couple of days somebody would start to catch on. Dang...
"Hey, buddy... how about a courtesy flush, huh? Did something die in there, or what...?"