There are secrets in the Bedroom, Slate and Kitchen maps.
Bedroom:
Go behind the pillow. There's a passageway to a spot inside the bed. Yay.
On the far side of the dresser (Facing the wall) there is an opening into the dresser.
Fly really fast into the window to break it and go outside. There is a TREE AND GRASS! HOLY stuff
Kitchen:
You can't ram into windows and break them here :(
You can go to the table area, and from there head to a small opening in the cabinets and then go into the cabinets.
There are other secrets in the Bedroom map.
Bedroom:
After you break through the window, there is a hole in the roof that leads to the two upper drawers.
At the base of the house, (on the outside) there is a crack hidden by grass you can crawl through, that will eventually lead to a secret building area.
Ikea House v2: There is a hole in the vent that's either over the oven or the sink that leads outside.
Can't look for more as my computer's crashing on certain maps.