Have you stayed at a hotel with such devices in the mini bar? Manufactures of the device has entertained all such scenarios which hotel guests will come up.

Hence, it detects any weight change. You are only thinking when items were taken then weight should be zero. But they have thought about all those scenarios, including guests like you replacing with a bottle filled with water to replace the items taken.

That is why those devises are set to monitor any weight changes, and some hotels put in writing that if there was anything detected by such device in the mini bar then guests will be charged regales of items consumed or not. Since a can of beer is cheaper if bought from local stores, some guests thinks just take one from the mini bar and drink now, later replace with exact same beer bought from store and all will be fine. Well, all will not be fine.

I think one of the reason such devices were created was that it was not that unusual that a guest will fill the empty bottle of mini whiskey with iced tea or water, capped it and return into mini bar. Such doing by guests not get discovered till way later. Such is the reason some hotel say any weight change detected will result in mini bar charge.