BL412 seems a good one as very simple. The circuitry around HR-501 is simple enough but quite a few components as I want to embed the circuit into my PCB, however it does have more options to adjust sensitivity etc. I was also checking Panasonic EKMC1601111 which is used on the Scope Clock Due that I purchased Grahame (Sgitheach, I believe he's in this group too). It doesn't require any ancillary components (other than a bypass capacitor and a resistor), but it's quite expensive about about $10 or more.
I also find interesting the microwave sensor (plenty of options, I was checking HW-MS03), although it require very careful PCB decign (what a piece of engineering that is!) and I suppose it means that the enclosure of the electronics cannot be made of aluminium or stainless steel. This has the advantage of being more sensitive although microwaves travel across wall so perhaps too sensitive for a clock.
Struggling to find the balance between simple design without it being overkill and something that does the job well.
So many options.