Some of you may have read about the cat that was found yesterday with injuries that made the owner suspect abuse (thanks E_Cat for the heads up too). The poor cat was put down - the SPCA vet has said that they are unable to ascertain if it is abuse, or if the cat got into an accident or fell from a height.
It is always safer to keep your cat in and safe in your flat. Outdoors, especially if it is wandering around within the block, it is subject to possible abuse and accidents. I would venture to guess that pet cats let out to roam are probably MOST at risk - community cats that live in the open downstairs have a better chance of running and hiding, but it would be more difficult for a cat caught on a high floor of a HDB block to get away because it is more enclosed.