I had a feeling this would happen but here I go.
There are a few clues to where it is. The memory cloud in the bathroom says "But I sealed the entrance in the kitchen," and another one says that "it can't get out of the basement." I think another one says "It's behind the wall, you wouldn't notice it at first." This should imply that there is a hidden wall in the kitchen.
If you still can't figure it out, it's between the door to the entrance hall and the garage. You can actually spot it without the hammer, but you won't see the eye icon appear when you hover over it. If you click on it, you'll see a message say "It looks hastily plastered over."