Simon is in the UK, camera central, with more cameras than people. Does not seem to help. The CCTV is good, they give great evidence, though you will need 2 systems, one visible and one to watch who vandalises it that is well hidden. The recorder will have to be well hidden as well, or at least the one that works, so that if the scum break in and nick it they get a broken one only and are on candid camera as well.
The car alarm systems with touch sensing use an IR transmitter and are mounted on the rear view mirror, so that as you approach they trigger on the larger return pulse after the gate time that ignores the windows. That should be silent, with a remote warning that it is triggered instead of a loud siren. Siren is for regular alarm.