Sometimes I wonder what ever happened to professional courtesy. Lately I've discovered that agents have been showing my vacant properties without scheduling a showing. When I call them to question them, they respond "I didn't think you'd mind or I tried to get a hold of you".
I make a point of making myself available by carrying my cell phone with me everywhere but the shower and church so I don't buy into the "couldn't get a hold of you" excuse. I wouldn't dream of entering a property without requesting a showing from a listing agent. I would never want to interfere with the possibility that another agent has scheduled a showing at that time.
There are circumstances where on the way to show another property you can come across a vacant home the buyers would like to see. In that case, call the agent and request a showing. Don't just assume that because the property is vacant, you can go in whenever you want. The property may already have an accepted offer on it or the seller may be taking it off of the market. A sign in the yard doesn't mean it's available. Sometimes we can't remove our signs until the spring.
Most agents in my area are fairly responsive and will get back to me fairly quickly if I request a short notice showing. I keep a list of the various real estate companies and their agents in my car so that if I can't reach the listing agent, I can at least get in touch with the on call agent or the broker.
Most buyers appreciate your efforts if you exhaust every possibility in an attempt to contact someone. They understand they may have to wait a day or two and realize that there are procedures that have to be followed as a professional coutresy as well as for security and ethical reasons.
Obviously, I want as many showings on a property as possible but I need to know by who and when.
Take care.
You have a valid gripe. If they do not make appointments and something happened to the home and their lock key was the last on the meter, seems that would be a point of possible liability or at least could be questionable. So not only for your sake but their sake also--make those appointments!