Maybe a recording like what plays here on the base loudspeaker throughout the day. "You have entered a secure area where deadly force is authorized" .I was thinking about this. What about a motion detector that was set up to play audio?
MicroSound Voice Motion Sensor - Waytronic
MicroSound Voice Motion Sensor Description: Motion sensor audio player ' s detection range is up to 13ft(4m), 120° view angle. It will trigger the soundwaytronic.ca
I SEE YOU ON MY CAMERA! LEAVE THE PROPERTY OR I WILL CALL THE POLICE
4 Pack IR Bullet Fake Dummy Surveillance Security Camera CCTV Record Light S1-4 | eBay
Cheap and effective way to deter criminals, installed quickly and easily to the ceiling or wall using the included screws, No wiring is required,2 pcs AA batteries operated (not included).www.ebay.com
fake surveillance cameras