And what makes you think this?
I think the doors were simply unlocked, so getting in could be done quietly.And with living arrangements in these homes, I don't believe noises would necessarily be worrisome to those inside.
Unlocked at CR's, but we know he was up. Then the killers stayed there until GR came home, when they caught him by surprise.
I think they took more time in CR's, then headed to FR's later. Once they got to FR's place, the killings went fast. In my opinion, this door was unlocked too. They simply crept in, killed FR first and HG woke up, which is why she took so many shots.
At DR's, I think that since they were smokers it wouldn't be a red flag to hear the door opening, someone may be having a late night smoke. I think HR caught the first shots, then DR, and I think DR woke up due to the amount of shots and the chin execution. I think CR2 heard this and was alert when he was killed, which is why he's found between bed and wall.
KR's was unlocked, I can't be convinced he didn't awaken the second his door opened. He may have even thought DS could be coming in.