A concealable burglar alarm system for automatically generating a warning in response to either unauthorizd opening of a window or breakage of the associated window pane. The system includes a translucent, generally rectangular sending unit which is adapted to be permanently secured upon a window sill adjacent to a window being monitored. The sending unit includes a generally planar mounting base, and an associated magnetic reed switch having a stationary portion and a pivotally deflectable portion. Relative displacement between the reed switch moveable portion and the stationary portion causes an electrical circuit to close, activating a remotely disposed electronic circuit. The base of the sending unit preferably includes an integral, outwardly extending cylindrical projection upon which the coiled portion of a reed switch activation spring is mounted and flangeably secured. The spring includes a pair of outwardly extending first and second ends which are biased apart. The first end is preferably permanently coupled to a portion of the sending unit base, while the second spring end includes a tab portion adapted to be user-coupled to the moveable reed switch member enabling the sending unit and thus the alarm to be mechanically activated or deactivated by the user. A translucent linkage member adapted to be glued to the window pane extends downwardly into contact with the moveable portion of the reed switch to normally bias the sending unit to an "open circuit" condition. However, if the window pane is destroyed, or if the window is opened, the spring will in effect "close circuit" the reed switch and a suitable alarm, preferably disposed remotely from the window, will be activated in response thereto.