DD15 Oil Cooler Replaced, and found this inside...

I have a 2011 DD15. 980,000 miles. Oil cooler o-rings needed replacing. Fixed the truck, but something went wrong. Not sure how, but I pinched an oring during reassembly and when I fired it up, I was pumping oil into the cooling system 10 times faster then before. Took it all back apart, and as I was removing the brand new oil cooler, I caught a glimpse of something in the hole on the cover. The right side hole in this picture. Again, it was like catching something out the side of my eye type of deal. I set the cooler down, and grabbed a flash light and found this black cap thing in the hole on the right. It had appeared to fall out of the oil cooler module and into that hole as I was picking up the oil cooler. I do not know what or how it got there. I do not remember seeing anything like this during disassembly or reassembly. I am at a loss. I am pretty sure it was not there when I put it all back together the first time. It is hard plastic. It has that look of well heat cycled plastic that has been in the oil for a long time, like several hundred thousand miles kinda look, so I'm wondering if it was in there prior to me doing this job to begin with and I am just getting lucky that it managed to work it's way to where I found it. Does this thing look familiar to anyone? It looks alot like those soft silicone covers you find on the end of a new bolt and nut. Like a thread protector or something. But this thing is rock hard. Anyone have any ideas?

 

It does look like it could be the tip of that piece in that second picture, doesn't it...
I'm headed down to the dealer tomorrow morning to pick up a new gasket and a couple o-rings..
I was going to take that piece with me, i'll have them pull that piece in those pics and compare. Unfortunately, I am not near the truck at the moment to go pull the oil filter out and see if mine is missing that top piece on it. This does seem to make sense. Would also explain the heat cycled feel to the plastic.

 

Case solved.. thank you swm_tech