I would have to compare it to the dipstick. when i did the exhaust headers i did have to remove the dipstick and it did snap but i was able to remove the piece but that does not mean the previous owner did not encounter the same issue and have that fall in. your suggestion sounds plausible. and it does have a magnetic drain plug which is what probably helped extract it eventually. i do not think those magnetic ones came factory.

 

i think you hit it on the nose. it definitely looks like the bottom half of the dipstick below the o ring. and most likely they snapped a tap trying to retrieve it exactly like you described.

 

Good to hear. It's been awhile since I did mine, but I thought it looked pretty similar.