Most consumers know that if they drink anything from the minibar in their hotel room, they’ll be charged. But did you know some hotels now automatically charge your bill if you as much as touch the snack tray — even if you don’t eat anything?

Automated minibars with sensors and snack trays with built-in electronic scales are now common practice at hotels, including Hilton, InterContinental, DoubleTree and Sheraton. But a slew of complaints in hotel reviews online reveal consumers still get taken by surprise when they discover “incidental” charges on their bill for food they never consumed.

A manager at Milwaukee’s Ambassador Hotel estimates that as many as 90 percent of automatic charges for the minibar turn out to be in error. There, a staff member manually checks the bar in each room and corrects any errors before the consumer is charged. But at other hotels it’s often up to guests to discover the false charges.

A reporter who visited a DoubleTree hotel in downtown Chicago at the end of August discovered six charges marked “minibar” on his bill. They ranged from $6.06 to $7.72 each. The reporter was puzzled because he and his family didn’t eat anything from the tray and hadn’t seen any signs warning that they’d be charged if items were removed or moved around.

When he inquired at the front desk about the unexpected charges, he was told the hotel automatically bills guests if an item is removed from the snack tray, even if the guest puts the item back. A hotel rep agreed to remove the charges.

The Milwaukee Journal Sentinel contacted a spokeswoman at DoubleTree by Hilton to find out whether the apparent lack of a warning note at the Chicago location was a mistake and to ask why the automatic system was implemented. A Hilton spokeswoman would not answer those specific questions, but acknowledged that hotels in the chain have the option of installing sensors in their snack trays.

“A small number of our hotels have units that do use motion-sensor technology, and should include information on the menu card or unit stating the way in which the sensors work,” the spokeswoman said in an email.

In automated minibars generally, each snack spot has a sensor. Items are typically charged to a guest account if an item is moved, although a charge sometimes occurs only when the item isn’t put back within 60 seconds. That’s potentially problematic for guests with curious kids or those checking nutritional labels for calorie content or allergen information.

Guests at the DoubleTree by Hilton in Chicago have complained about automatic charges at the hotel going back to at least 2010.

“The snack tray was horrendously expensive ($7 for a little bag of candy) and God forbid you touched anything because the sensors in the tray would automatically charge it to your credit card,” a Canadian guest said in an online review on TripAdvisor in December 2010.

In response, a DoubleTree representative wrote: “Our mini bar and snack tray are sensory activated so if a guest happens to pick up an item for a few seconds, they can erroneously be charged for something they didn’t eat. If something like this occurs, we just ask the guest to be honest about the mistake at check-out.”

Amy Schneider, assistant general manager at the Ambassador Hotel in Milwaukee, wasn’t surprised to hear about guests at other hotels being unfairly billed for food they didn’t eat.

“That’s dishonest. If they don’t have a sign, that’s a level of dishonesty,” she said.

Schneider said Ambassador implemented an automatic minibar as an amenity to high-end travelers during a renovation four or five years ago. Guests initially complained about automatic charges, but the hotel has since adopted a system in which an employee manually checks that items were actually consumed or opened before the guest’s credit card is charged.

“Nine times out of 10, it just means something was picked up and put back,” she said. “No computer is a fool-proof system. You always need human interaction.”