I would hate to get a felony on my record. The Subciety site,
http://www.subciety.org/security.html , states "Trespassing can be a felony in most states." However, Washington state law (9A.52 RCW,
http://apps.leg.wa...lt.aspx?cite=9A.52 ) says it is a misdemeanor (on property) or gross misdemeanor (inside building). It is a defense that the property has been abandoned.
Cautiously, I looked up the legal definition of "abandonment,"
http://legal-dicti...ry.com/abandonment . It's not specific to real property (land), and it's not unequivocal. As encouraging as it gets; "A person's intention to abandon his or her property may be established by express language to that effect or it may be implied from the circumstances surrounding the owner's treatment of the property, such as leaving it unguarded in a place easily accessible to the public. The passage of time, although not an element of abandonment, may illustrate a person's intention to abandon his or her property."
Subciety claims they never enter a site that's clearly posted "No Trespassing." That seems reasonable if you want to obey the law; such a sign would indicate that the owner does not intend to abandon the property. I'm guessing that going past the sign would activate the "knowingly" condition in the (Washington) law.
Caution; I'm not a lawyer, just a guy who can google and read.