Hi Buddie,
Welcome to our forum. ;D
Hardware stores have a pretty good PIR motion detector sensor that can trigger an alarm of your choice.
At work, they got a wireless PIR motion detector with a wide sensing span and a range of about 10 or 15m. Its wireless range was far. It was from RadioShack. Its alert tone was an awful-sounding abrupt square-wave so I filtered it into a pleasant-sounding sine-wave chime.