If you are worried enough to ask the question here you're worried enough to make a welfare call.

Call it through to police on 000 - it might not seem like enough of an "emergency", but it is something that gets organised via 000. Don't worry you are wasting police time, they do a lot of welfare checks and there is almost never something actually wrong, but some of the time someone has fallen and is lying on the floor in desperate need of help and organising the check saves their life.

Go knock on the door and explain that you were worried as you hadn't seen them in a while.

Check to see if mail is piling up and whether bins have been moved in a while, knock on the door a few days in a row maybe, see if you see or hear anything.

And don’t be afraid to call.