Breaking and entering...
well, first I'll say that I'm not a lawyer. I have, let's just say, a bunch of personal experience in this area.
First, that expression "if you're going to break the law you should know what it is" definitely applies here.
To that end, I'd recommed going online and doing some research before setting foot onto the property that you're looking to target. Is it a private facility, or state owend? This can make the difference between a misdemeanor, and a felony.
Since I live in CT, and alot of what I see posted here is about CT, I'll recommed going to:
http://www.cga.ct.gov/2006/sup/Title53.htm to look through the CGS (CT General Statute) regarding B&E, Trespassing, etc.
Secondly, b&e doesn't necessarily mean that you were the person who broke in. Someone broke, but you are the person who in fact entered. If caught inside, the assumption is that you were the person who "broke." It wouldn't matter if this was done 5 minutes or 5 years before you got there. Especially if you consider that most if not all these abandoned site have clearly posted "No Trespass" signs, the defense "I didn't know" is out. If caught inside, you would take the brunt of the responsibility, legally, for whoever broke in. You can tell the judge "I didn't break in, someone else did," but that means little in a court of law.
Under the eyes of the law, it does make a difference if you're caught inside, or outside of the buildings. It's the difference between simple trespass (mis.), and Criminal trespass (Fel.). Also, at the time of "arrest" if you're found with certain tools, that would be viewed as "attempted Criminal trespass" because you are viewed as having an intention of B&E, or some other crime against the property; whether or not you did in fact have that intention. Especially on State property, having those tools would essentially carry the same penalty as if you actually were caught inside.
On the flip side of this once the cops are called, you don't have to tell them anything. It's up to a judge, and your defense lawyer as to what the penalty will be.
So take all that for what it's worth--and of course, DGC.