ZC and/or her roommates could have heard the buzzer (and looked at the time) then put 2 and 2 together when the timeline was reported.
Just bumping up an old article...
http://www.idsnews.com/news/story.aspx?id=85908
Signs or no signs, people aren't going to forget about Lauren, and this case isn't going away.
And we will all make sure of that!!!
But, I do think, as long as she's still missing, why take down the signs, bad for "business" I guess, which I find disgusting. I'm hoping that those signs are continuous reminders to not forget Lauren and be safe and know who your real friends are.