I've been to a few E3s, there are security guards at the doors that check your badge when walking into any of the halls/upstairs meeting rooms. I'd say the security is decent but not super locked down - pretty sure some people have managed to sneak in years past.
But yeah, the parties are an entirely separate affair, and aren't held at the convention center anyway.
But yeah, the parties are an entirely separate affair, and aren't held at the convention center anyway.