Well, now that you guys mention it I think it is the bolt that retains the handle on a floor jack.
But, I have seen drain plugs in engines that look a little like that. The nipple is actually a magnet that catches any metal filings that fall into the oil.
Since it is not oily, that is not what it is. But, it looks a little like one.
By the way, if you take it back to the shop where you had the work done they will thank you. Then they may like you and treat you better then next time you are in a predicament .