I'm no legal expert, but I was curious. One definition of trespass:
"Entering another person's property without permission of the owner or his/her agent and without lawful authority (like that given to a health inspector) and causing any damage, no matter how slight. Any interference with the owner's (or a legal tenant's) use of the property is a sufficient showing of damage and is a civil wrong (tort) sufficient to form the basis for a lawsuit against the trespasser by the owner or a tenant using the property."
Says AND causing damage etc.. Anyone should know that causing damage is wrong.
Hunters and land owners have going through all this for ages. Generally, the area has to be posted. Clear signs spaced a certain distance. (80 to 100 feet?) In some states they must also have the owner's signature on the sign.
If you are on an unposted property and are asked to leave immediately, you can be charged if you do not leave. Simply entering a properly posted property means you have violated the trespassing law.
That's just the letter of law. Sometimes the law is made up on the spot by whoever has the badge.

"Criminal Trespass", here's one definition:
"While the precise definition can vary from state to state, a person commits the crime of “criminal trespass” when she enters or remains on another’s property without the owner’s consent. In Tennessee, the law assumes that the person knew they didn’t have the owner’s consent if the owner or someone with the authority to act on behalf of the owner personally communicates this fact to her, or if there’s a fence around the property, or if there’s a sign or other posting on the property that’s likely to be seen by intruders.
You may have a defense against criminal trespass if the property was open to the public, or your conduct didn’t substantially interfere with the owner’s use of the property, or you immediately left the premises when requested."
B&E:
"entering a residence or other enclosed property through the slightest amount of force (even pushing open a door), without authorization. If there is intent to commit a crime, this is burglary. If there is no such intent, the breaking and entering alone is probably at least illegal trespass, which is a misdemeanor crime."
I think pushing an unlocked door is a bit extreme, but back to who has the badge....

I'll also add that I wouldn't appreciate people poking around my property and buildings, unlocked or otherwise. Good way to get hurt. Best to ask permission, even when hunting, picking mushrooms or berries, etc..
http://legal-dictionary.thefreedictionary.com/trespassinghttp://criminal.lawyers.com/Criminal-Law-Crime-Definition-FAQs.htmlhttp://legal-dictionary.thefreedictionary.com/breaking+and+entering