Nice warning. Just because you have a 'trespassers will be shot' sign, that still doesn't mean you can start shooting trespassers. The US courts have come down heavily against photographers who have tried to claim large damages against companies for wrongful usage of images. In the last case I heard of, a photographer sued Urban Outfitters for using one of his shots without a licence. He not only lost the case, but had to pay $388,000 in costs, and had his lawyer censured.
Go to the
PDN web page, and at the top of that you'll see a link that says PDN Newswire. Under that, there's a section on legal news. Read through some of that and you'll see just where you stand as far as the courts are concerned.
David Buzzard