The foil can be defeated by taping the glass very heavily on the
outside and most of the time it will not breach. A detector is more
fail safe.

The other thing the detector affords is the ability to actually go off
without a breach. It can detect the act of attempting to break the
glass where the first hit may not penetrate. It can also monitor more
than one pane. In a corner or placed at an angle with deflector behind
it, the detector acts like a PZM microphone.
--- SBBSecho 2.12-Win32