Well.. not if you lived alone like NG did, she would place it somewhere easily accessible I'm sure, thinking for herself or family. It is cliche.. but happens all the time sadly.
or.. due to the amount of time before anyone noticed, the perp came back but he already took out the front door camera, one they knew of, so after leaving with NG and coming back without her to check things, would pull it up if a key was there. Knowing no camera isn't there anymore. Diposed of it to get rid of the "idea" of a key being there, meaning someone close.
It's plausible they came back, with the amount of time.. could've been more evidence but didn't have time and decided to come back, but it would've been a bad idea to go in the same place you went in previously (ie back, etc), knowing of the "Key" in the pot, grabbed it because you orginally locked the door leaving with NG. This would throw investigators off a bit, they are probably thinking its from a scuffle.