As Nic suggested to me last week, these builders clearly believe the assumption that no one can see them, due to the 2m fence around the site.
They must think it’s a +4 Cloak Of Invisibility(tm).
If they thought anyone was watching, they wouldn’t do things like spend hours walking around the site staring at the mud, occasionally kicking stones to see if they’re bolted down or not. (To date, he hasn’t found any bolted down — Ed)
What they don’t appear to know is that people in the surrounding buildings have a far superior Cloak Of Invisibility(tm), resulting in us being able to watch them, without them ever realising it.
They remind me of human implementations* of The Ravenous Bugblatter Beast of Traal, with which I assume you are all familiar.
* Not 100% sure that’s the best word, but it’ll do for now. Suggestions on a postcard, please.