Ok, so I'm not new to guns by any means.  But I cannot figure this one out.

Took my P3AT apart to clean, and when I turned my frame upside down, this little plastic piece fell out of it.  I had not taken anything off of the frame, just removed the slide.  I cannot figure out where it came from  -  no where it looks like it belongs and I cannot find it on any of the parts schematics or referenced in any disassembly video or post.  The gun went back together fine and seems to be operating normally  (I haven't shot it yet, but as far as I can tell, it's working fine)

Oh teh noes, that is the Full Auto rate reducer. Without it the gun will empty the magazine in 0.000456 seconds. But the good news is that it will happen sooo fast no nearby RO's or ATF agents will recognize the sound and you will be off scot free.

Oh yeah I remember that part, I think it goes into the frame if I remember correctly. Take your gun apart and see if you can't fit it into the frame.

So did you put the part back in.  Did you read the manual it clearly explains that may happen.
Page 13, "CAUTION: Once the slide is removed, the Ejector (part #115) will easily fall out."

It's the ejector. It fits into a little slot in the slide rail on the left side of the frame.

Perfectly normal that it fell out, but you'll need to put it back for the gun to function properly.

*sigh*

now I feel like an idiot.  I never thought to look on the slide rail like that.  

#ThingsYouLearnFirstTimeCleaningYourGun