The Powered alarm would give out a loud sound when powered and receiving a Redstone signal. The sound could maybe be selected by the player via a GUI or right clicking with a wrench.

The detector will draw a small amount of power constantly but will give off a Redstone signal when a target is detected in its set range.
The target and range for detection would be modifiable.

These 2 blocks combined would allow for a whole host of create security options from tesla gates to hidden security scanners.