The company that came up with this says,
"Tired of having your food stolen by sticky-fingered coworkers or roommates? Bullies taking your kid's lunch? Well, worry no more. Anti-Theft Lunch Bags are sandwich bags that have green splotches printed on both sides, making your freshly prepared lunch look spoiled. Don't suffer the injustice of having your sandwich stolen again! Protect your lunch with Anti-Theft Lunch Bags."
Thanks for the link to Jeff Olsen, who asks, "Wouldn't the thief still take your lunch, and then throw it away?"
If you do buy this product, you'll have to stop cutting your sandwich, or you'll ruin the effect when the thief sees mold in the gap between the two halves.