Can you put up a notice saying "please return the computer that was accidentally left here, no questions will be asked"?

Hopefully your brother won't follow through with his threat to have you arrested for theft, when you do not have his computer!

I'd suggest your Dad, maybe, walk around the neighborhood and humbly ask if anyone knows who picked up the computer he accidentally put out to the curb, as his son is very upset with him for doing so. You never know, someone might know who took it!