- to look by something = to look near something
- to look past something = to look beyond something
- to look around something = (usually) to look within something; (rarely) to look beyond something
If you need to find something and are told to look by the house, you should look for your missing object in the house’s close proximity — that is, to search for it near the house.
But if you are told to look past the house for it, you have to keep going beyond the house to something beyond it before you can look.
Interestingly, to look around is also open to multiple readings, although by far the most common sense is more like look by than look past. You look around your house for something, or you look around the back fence. (If someone is just looking around, though, they are casting their gaze in many directions.) Only rarely does one look around an obstacle blocking one’s view. Normally one just looks around a house or a garden or a backyard, where it means within.
Also, to look around for something is different again.