Hotel guests have left all manner of personal items behind in rooms at the Fairmont Hotels & Resorts chain—pillows, false teeth, chocolate-covered ants, a book on how to improve your memory.

But nothing compares with mobile-device chargers. The Fairmont Waterfront in Vancouver, British Columbia, has acquired about 300 of them. In California, the Fairmont San Jose is left with about 250 chargers a year. Maids at the Fairmont St. Andrews in Scotland find as many as 35 a month still plugged into wall outlets.

"They're...