Ahh these are too funny, I love the ketchup and mustard one too! Our office is covered in stuff like this, we all have to mark whatever food we put into the fridge because people just steal it.
My colleague and I used to buy a little carton of a certain type of milk for our morning tea every monday morning and one Monday tea break we went to the fridge and our brand new milk was all gone and the carton in the bin, so my colleague sends round an email to the entire office asking the person who stole our milk to replace it. When it wasn't replaced she sent round another one basically telling the person he was a douchebag for stealing someone's milk. Our director took pity on us and bought us a big bottle of milk hehe.