I don't mean to start a thread on this, but see the third bullet from the Jeffco Sheriff CCW web site. So if the owner of a business has a posted sign - not supposed to carry there:

12. Where are the carry restrictions?
A person may not carry a concealed handgun into:

* a place where the carrying of firearms is prohibited by federal law;
* a public building at which security personnel and electronic weapons screening devices are permanently in place;
or
* where a private property owner, private tenant, private employer or private business entity disallow.