Sirius' mind is not necessarily on his personal safety, at the moment. It's dangerous, with a war on, but he's got a pregnant girlfriend and a werewolf boyfriend lost and now returned and two other partners in their own levels of panic. It's distracting.

It's got him careless.

So he isn't as sharp about checking the wards as he should be, when he gets home. He isn't looking out for any signs he isn't alone when he opens the door.

Isn't at all thinking to scan the room or properly turn on the lights before he settles properly into the flat and locks the door behind him.

All of them are mistakes. Mad-Eye Moody would probably be furious with him, if he'd known.

Too late to fix it now, though.