I wondered if someone would ask this. I discussed it with my lad only last week - he works at the same place.
I've mentioned to various people over a long period of time, the dilemma that one faces when choosing a cubicle. Common perception is 1 or 4. I can see the sense, you can't possibly have someone on each side of you should the worst happen and there's a rush.
Now, in a similar way to Pep's issue with the attendance last night but with a different desire for hit rate - it's all about bums on seats. I think that less people are likely to go 2 or 3 so plump for 3 as long as it's clean, which it invariably is - therefore supporting my theory.
There's still a trap unoccupied two doors away so if another shitter comes in, they can still obey the unwritten rule.
What was possibly even more peculiar about the weirdo this morning, was that he had his shit, then stood up and pissed on it.