This is an interesting side question, the frequency of food tampering at restaurants and fast food places. I have worked in a number of kitchens and I can confidently say there was never any incidents of food tampering, nor have I heard of stories from any colleagues in regards to them or their coworkers willfully tampering with food.
The closest I have experienced was when a customer at McDonald’s complained that their burger was not picture perfect. Well, duh, that’s what you get at McDonald’s! So the back crew started making him a new burger, being over the top perfect, six guys working on one pickle etc, all the while talking loudly about Mr Perfect Burger. Of course, Mr Perfect Burger then wrote a letter complaining about the incident (side note, Mr Perfect Burger was a scam artist who frequented the McDonald’s I worked in and consistently [we’re talking once a MEAL on average] complained about the smallest of non issues just so he and his family could get free food. The best day of working there was when my mentor manager got so infuriated at MPB’s menial complaint and the brown nosing “supervisor” who commanded him to obey MPB that he ripped off his shirt, quit his job, and challenged MPB to a fight.), and there were some private discussions with the boss, but there was no food tampering and no real punishment inflicted. This is one incident where a bad crew member could have easily defaced the burger, but they did not.
Maybe it’s just different here in Canada, but I have no worries ever that my food will be tampered with. Then again, I’m not a food purchaser who calls and complains and bitches about every single small and trivial issue.