I think what they were trying to portray was the concept of finally being able to rest; to let ones guard down after some harsh days. By this point they'd been on the road for days, moving a lot and sleeping little. I think in the timeline thread, we'd established that this was probably at least 5 days in, keeping in mind that we've no idea how long Daryl was teaching Beth tracking before she stepped in the trap, so it could be longer. Unfortunately, while I get what they were going for, I don't think it came across well, as everyone seemed to find their behaviour at the mortuary to be unlikely (well, Daryl's, anyway). Or maybe we're overanalyzing, and just need to accept that even the most competent might eventually drop the ball once or twice.
Poor Daryl - first time he buggers up and we're all over him for it
ETA: I don't think anyone is necessarily surprised the walkers showed up, with the music and the lights; but it seems extremely odd that they'd be so quiet. They've only really been shown to be that quiet when there is nothing attracting their attention.
EATA: Wasn't the piano playing the previous night? The night the walkers showed up they were just sitting in the kitchen, eating and having a 'moment'.