People in LE are not superhuman nor do they have endless resources. These investigations can take a while and companies who have analysts with accounts on all such sites to monitor this sort of hidden(ish) chatter are very expensive. I doubt the underfunded and understaffed UK LE for example can afford to hire a boutique consultancy just for MH related investigations.
In theory you can put together a script to crawl this site and narrow down the Interpol list i.e. if some mentioned they are in Essex area here and there are only a handful people on SaSu from there but even then, you probably need permissions to do it, you need to involve the council, the ISP, get the necessary greenlights for this to hold any weight, and even then, now you sort of know that people who bought suicide kits post on suicide forums. Then what?
