Explain Security System

There is little info on the security system in the owners manual.

Last night I went into the garage to put something in the trunk, and it wouldn't open with the manual button under the Buick emblem. I've done this many times before. Car wasn't locked. Thought that was odd, so I went back in the house and got the remote key. It opened fine with the button on the remote. I put the item in the truck and then, for the first time ever, I noticed the bright flashing red light smack in the middle of the top of the dashboard.

I opened the passenger side door and it quit flashing. Also I could then open the trunk with the manual button. Hasn't happened since.

So what happened?

Also does anyone know what the small push button is that is right under the southwest side of the speedometer? Again, nothing in the manual. I assume it is to reset the trip odometer or start the timer. But I haven't figured out how to do it.