Can I do free-air sniffs of vehicles on private property?

No. Private property implicates the Fourth Amendment and residents have an expectation of privacy in private property. However, if you are in a public place or in a place where you have permission to be with your K9 (such as a neighbor’s yard with neighbor’s consent), a free air sniff will probably be permissible. This is an issue that is really driven by the fact situation of your case.