Pull the photographer gig on the guard and see what you can find out. Best case scenario you get a guided tour, worst case scenario he tells you to fuck off but you get some clues about the site's status. Go back later, avoiding security. Someplace this big there's no way they could patrol it effectively without several guards, and who's going to pay for that?

i heard (from a park police officer) that they recently decided to set-up a 24h security guard for the building after a kid recently fell-off one of the fire escape ladder and suffered very serious injuries (head injuries, maybe broken skull).

The signs "no photography allowed" and "absolutely no tresspassing, property of the FEDERAL GOVERNMENT" have always been there, you can ignore them, this building is an abandoned ruin and has been abandoned for 25 years or so, and i doubt very much that the federal government would seriously prosecute people who decide to go in there to visit the place or take photos.

i have published on my website hundreds of photos taken in the building, and i'm not worried about that at all. when park police catch people in the building (or getting out of it), they write them a $100 ticket for misdeminor trespassing / violation of park regulations, and this does not go on your record - it's about like a parking ticket. but it's always best not to get caught when doing urban exploration :)

I've been there. When you're driving in from up on the hill, you will pass by a park security guard. We were snooping around and when we got back to my car she was waiting for us. She just us we couldn't leave the car there. I told her we were just looking around, taking pictures, etc and there were no problems. She was actually very helpful and friendly--which brings me to a point:
---sometimes security can be your friend, they have access and you don't, so might as well make nice and see what you can get out of your encounter with them. that way they're less likely to call the cops too, because they know you're not tagging or anything. I always go out with my gf, so we look like a harmless couple on a weekend outing. on another occasion, down at the docks in oakland, the security guard actually let me and my buddy into the big parking lot by the box cranes and train stations despite all the homeland security restrictions, just because we were friendly and he was totally bored.-----
If you go down to the beach, which is a wonderfully deserted area in it's own right (someone once erected some sort of tepee down there that is now just a skeleton), you can access the old pipeline on one end of the beach and on the other you can walk along the water until you get to the fence to the federal buildings. roll up your pants and walk out into the water a ways and wah-la! This was a long time ago though so let me know if they've blocked it off. we were a bit paranoid, but it's not so bad. someone came along the fence walking their dog and didn't seem suspicious (again, look harmless and people will think you're harmless). Watch out for the tide! You don't want to fall over onto some barbed wire.

up the beach there is a great shipwreck and the views of the Richmond Bridge and Mt. Tam are glorious.

Afterwards drive up to the San Pablo Yacht Club. It is the most derelict Yacht Club I've ever seen. Definitely worth checking out: part salvage yard, part boat cemetery, and part housing project, some of the people that live aboard the boats are rather territorial and give you the stinkeye when you wander around.