It is. There's an OE community (Maybe a movement at this point). Where I work you're actually allowed, with permission, to have outside employment. But there was a case where a remote worker was found out to be working another full time job at the same time. They didn't have permission, and they were fired, even though their boss had no actual complaints. (I think it was discovered through that experien system that aggregates worker history & salary, which most companies subscribe to.)

It happens, it's real. And believe it or not, there are many "full time" jobs out there that can be done by a competent professional in half the time.

Yeah this can trivially be detected by a system similar to how certificate transparency works. Every company has it in their interests to exchange this data

You can freeze you experian data so no new changes are added. I just found out about it when I went down the OE rabbit hole to see if this was legit and I'm well and truly creeped out by what's collected there. I'm perfectly fine "noping" out of yet another massive corporation tracking me in depth & detail without my explicit approval, so I have started the process of getting such a freeze.