For now, it's in the hall closet outside our bathroom. Two reasons:
1) Kids. Our youngest two (but our toddler especially) have been perennially fascinated by the thing. There's no greater horror than seeing them walk out of the bathroom waving an unsheathed plunger and now knowing what they did with it in the minutes prior. Now you're probably thinking 'chemistk, why isn't your bathroom door closed?' - our oldest & his friends (when over) are horrible at remembering to close the door to the bathroom, so it's always open. It's bad enough when toys get tossed in the toilet but there's something extra-special horrifying than a dirty plunger.
2) Because of 1), normally we'd keep it under the sink with a lock but wouldn't you know it, our 1960's vanity is too small (floor of the vanity too high) to fit the plunger standing up. An accessible plunger is, currently, not worth the cost of a bathroom renovation.
In about a year, our toddler will be old enough to be instructed not to touch it and problems 1) and 2) will vanish and the plunger can go where most people have them.