You need to do whatever it takes to find that key. You don't want to leave it in the engine thinking it won't cause any harm. Like a previous reply mentioned there is a fine screen on the oil pump pick up, but after that key swirls around and bounces against that screen it will eventually get sucked through the screen and cause the oil pump to self destruct. It may not happen tomorrow or next week but it will happen eventually.
I have seen this happen several times over the years and it is not pretty.
Good luck, Robert