Had that with ketchup I used to keep in the fridge, half bottle went missing over the course of a week (It was labelled as 'Sams - Please ask if you want some'). Took home, refilled with ketchup mixed with dragons blood chilli sauce (Naga pepper sauce - very very hot), I like the mix.... Whoever stole didnt, hasnt been touched since even if it is just ketchup again now. Teams buy their own milk here, and again we have issues with milk theft. Adding a drop of green food dye didnt work (it got binned) so we now keep our teams milk in a thermos. Its easy really, if it isnt yours, dont take it.