I'm not sure if it's still the case, but I seem to remember at one time the postie would often keep a bit of an eye open for signs of problems with the older folk on their round, more normally at least around my neck of the woods it's something like a neighbour not seeing someone for a while and looking into it.
Going back many years my father used to do window cleaning/general handiwork/carpet cleaning etc*, and more than once when out he was asked by neighbours of his customers to have a look as they hadn't seen someone for a day or two and normally saw them daily, at least once he ended up going through a window when there was serious concern and the police attended with a "we're not allowed to break in as it's not been long enough since they were seen" attitude, but quite happy for him to climb in through the open bathroom window as long as he was willing to take the risk. IIRC it turned out on that occasion that the neighbour had in fact had a fall whilst out and was already in hospital in a different area (this was back before mobile phones/easy computer checks).
A few months ago one of our neighbours realised no one on the street had seen someone for a week or more and there was a lot of calls to/from other neighbours before contacting the police who did a check and confirmed the old guy was in fact in hospital, IIRC he'd been ill and taken from the doctor's to the hospital or something.
More recently the neighbour across the road from me had a nasty fall down the stairs, fortunately his direct neighbour heard the fall and called the police/ambulance immediately after not getting a reply, unfortunately the neighbour passed away in hospital from the injuries
Basically, as I think pretty much everyone says, it can be until someone notices that someone hasn't been seen for a while, and these days that might be days/weeks later, I've heard of cases where it's been months/years because all the financials were done automatically and it was only when something like the utilities went to cut them off/they didn't respond to letters from the council tax or similar that anyone looked
*He knew many of his customers for years, and always kept an eye out for them (I'm talking the 80's and 90's).