The way to get around that is to sneak the box into the house when your wife is in the shower. That typically gives me a good 45 minute window. If she does see it, it's your buddy's rifle and you are mounting a scope for him. Works every time!
One of my married buddies told me his system was to take an empty gun case with him when he went to a gun show. He said then if he bough something new his wife just thought he brought the gun he left with back.
I found it easier just to not get married in the first place.
I know a guy whose wife use to have an outfit addiction. She would buy a dress and come home and hang it in the closet for a month or two... When she'd get it out to wear it, he'd ask if it was new?