Thanks for the answer.
So I think it's a bug, although I'm not sure how much time I want to spend getting to the root of it. The "Silence of the Zombies", as nobody is calling it, started on my day 5 with nothing; no sleepers, no walkers. Quitting the save and reloading will allow the first "spawning" event to happen then no more after that. I did in the end reboot the whole PC which has resolved the problem (I've only played one more in game day so can't say if it's fixed completely).
Upon rebooting and loading in the save it looked like no sleeper zombies were being spawned at all. I ran, without a care in the world, right into the middle of a PoI and saw nobody, then... suddenly... they all sprung into action. This is normal behaviour to be fair, if you're quick you can catch them appearing on their spawn spot - or even run past them before they have spawned in. Of course in typical gaming fashion I had run into a PoI that spawned a bear (a normal one, not the Bear Den zombie bear) which trapped me in the room I'd run into.
