"he probably snuck in"
You say he chloroformed her and made off with her as she slept.
Ok, to all that, I say a door had to be unlocked, and he had to know where to go in the home to get her, and there was a dog (of unknown watchdog capability). Lots of "had tos" for it to work in his favor.
They say the dad claims to have found the front door unlocked when he got home at 6. If true, either the door was inadvertently not locked that night, and the perp seemed to know it, choosing that night to get in, or Jessica was awake and knew the person wanted access and granted it to him, probably because she knew him.
The police say there was absolutely no sign of forced entry.
Elsewhere on tv accts that it had a habit of burrowing into a pile of blankets, and that's where it was found when the grandmother made the 9-1-1 call.
"Luck is what happens when preparation meets opportunity." -- in bad things as well as in good, it would seem.
Inspector Gregory:
"Is there any other point to which you would wish to draw my attention?"
Holmes: "To the curious incident of the dog in the night-time."...
Maybe he had a key. Maybe he was a locksmith. He could have been somebody who had worked around the house, been inside, etc. Whoever he was, this thing didn't happen on the spur of the moment. This was a cunning predator who had probably been stalking this girl (her and her family's routines, what time the parents left, school routes, etc.) and scoping out this house for some time. He looked for and found a weakness and then exploited it.
It pays for us parents to always be alert to our surroundings. Paranoid as it might sound, I have had times where it seemed like a car was following me (I'm sure we've all had this experience) for an uncomfortable amount of time and down a lot of coincidental odd roads and I purposefully lose them and take another route before going home. Better safe than sorry.