Abstract: An alarm adapted for mounting in an interior corner portion of a display case has an L-shaped housing which encloses a battery, an audible alarm, a mercury activation switch and an enable/disable switch. When the alarm is mounted in the display case, end faces of legs of the L-shaped housing abut adjacent interior side walls of the display case, forming a protected rectangular space for the alarm enable/disable switch. The enable/disable alarm switch is turned on or off by a key inserted through a small hole drilled through one side wall of the display case. The key may be in the form of a cylindrical pin for a simple push button type switch, or may have specially formed projections for engagement with a cooperating recess formed in an alternative rotary type switch. The audible alarm is set off by a mercury switch. In a first embodiment, an interior conical recess in a block of a plastic material contains an encapsulated pool of mercury.