When we were selling my dad's house the building inspector and the buyer's real estate agent made a huge deal out of the lack of carbon monoxide detectors. "Installation" was mandatory, required "proof of installation," etc. The inspector did notate that he saw a detector in its package on a shelf in the laundry room. "Installation" involved removing said detector from the package and plugging it into an outlet. So, my brother took video of me getting the detector off the shelf, removing the package, plugging the detector into the wall outlet, and performing the test. The buyer's attorney insisted on additional proof. Our attorney said "no." The buyer agreed with us as did their mortgage company.