Was water added to the vodka bottle?!
Answers: besides not tasting as strong, would the inside of the bottle have moisture on the sides, and tiny bubbles when you shake it?
put it in the freezer
alcohol doesnt freeze, but water does. if any bit gets frozen than someone was sneaking your booze
Put in the freezer for a couple of hours(3 or 4)- the water will freeze and the alcohol will not. That's how my friends got caught when we were in high school!
freeze it . someone added rubbing alcohol to mine once , damn near killed me