Just be sure to wash the can of diet coke really well first! (Cans of pop often left on the floor of cellars - rodents have free run over them, and as you may know, mice dribble urine almost constantly as they move about.)

Of course it could be post mix... Which is lovely, if they actually clean the lines properly and frequently, otherwise the build up can be really nasty, and
you can get insect pests in the system! mmmmmm

There are times I really wish I hadn't spent time in kitchens / restaurants / cellars!!! But as my granny said, a little dirt never hurt! I still eat out several times a week, and am still standing!