Hello everyone, I am actually the creator of this website/map, I just stumbled upon this forum.
The map on my website is not fully complete yet, but I believe it is the most complete (publically available) map on the internet. A little more info; the sirens on the map were mainly "crowd-sourced", as anyone can fill in a form on the website to report missing siren locations, and a few people that already had their own collections of locations contacted me. In this way, we were able to nearly get all the sirens in the Netherlands in about a year. I now realise I maybe should have found this forum sooner, but there might still be locations that are not yet on the map.
Also, sadly the website is only in Dutch at the moment, I never thought it would be necessary to translate (and my website started mainly as a hobby project), so I hope that Google Translate will suffice for people that do not speak Dutch :)
Feel free to take a look, and maybe even report some missing sirens, even if they have been removed by now, which happens sometimes in the Netherlands.