Great response. I don't think a policy that is not expressed to the public is actually a policy. What good is a secret policy? I would never suggest that a business post a sign, but if a business wants certain actions excluded from their property, they have an obligation to tell people what those actions are. Lots of places don't post because they are afraid that doing so will cost them business, we need to confirm that suspicion. I think a complaint to management is required. I hope you know this guys name/position. Whether they post or not, they need to know that this "policy" will cost them.
Not particularly picking on Gutshot. Just using his post to work from.
I don't know that any property owner is obligated to post his rules in advance. Holding to a pre-posted view is essentially saying he has to think up all his policies and post them, and he's in the wrong if he fails to foresee something against which he would make a policy.
Basically, this is a timing matter. A sign notifies in advance. A verbal notification comes later.
Now, here's a bit of a criticism. Basically what is occurring in these situations where an OCer is verbally notified and the OCer asks about a sign is that the OCer is being defensive. The "where's the sign" question is a defensive reaction. Why be defensive? He didn't post a sign, so its not your fault for not seeing it. And, if he did post a sign and you missed it, so what? Its not like you failed to let a little old lady get fully across the street and splashed her with mud. Moreover, and my biggest criticism, is that the "where's the sign" question shows a lack of consideration for the property rights of the business owner. If one really, truly respected his property rights, one wouldn't require him to clutter up his property with one more sign, or spend money on a sign, in order to ward off the occasional OCer. And, perhaps most importantly, one would recognize his right to make up a new policy on the spot and notify verbally.
The owner/agent can tell whoever whichever whenever.
So, don't get defensive if verbally notified. Don't ask about a sign. Just take the high road, be polite, make your exit. If you want to get in a counter-dig (which is your right), you can always point out to him that you won the instant he opened his mouth--you were willing to respect his rights, even though he wasn't willing to respect yours. In fact, I would at the very least say something like, "Oh, sorry, sir. Yes, sir. I will respect your rights even though you won't respect mine. Have a nice day." ("Thanks for giving me the win" would be optional.)