It reminded me of this thread:

I've read one other instance of this happening but I don't remember if it was here or on Glocktalk.   I also want to say that mine has not fallen out, I've only noticed it while cleaning.

I apologize in advance for the crappy focus.

There was enough play in the lock that I could move it freely to this point:

Either side would go down that far and no farther.  

So, I took the slide off and noticed the spring sticks up just above the surface of the frame.   If I set the recoil spring down inside the frame, it doesn't make contact with it unless I apply downward pressure on top of the slide, like would happen with the slide locked back and the barrel dropped down.

I also noticed that I could push it down farther than it goes when the slide is locked back.   I tried to put a different spring in the receiver but it didn't change anything.   I didn't notice any difference between the spring in the new gun and the spare I purchased four or five years ago.

It will not come out unless the spring is compressed farther but I'm curious if it could in the future.