Funny business

A: Hey, the deal that company made doesn’t seem right to me.

B: You think there might be some funny business going on behind the scenes?

Note: The idiom funny business refers to any suspicious, unethical or illegal activity, suggesting that there is deception or mischief involved. It was initially used in the context of organized crime, but has since expanded to describe any questionable behavior, whether in business, relationships or other contexts.