Question re: gifting money

If you gift somebody some money, I know that as long as you live for a further seven years then no inheritance tax is due on that money.

My question is, if you die within the seven years, how do the hmrc know that money has been gifted? Is it down to your executor to inform them? If so, what happens if the executor is unaware?

I have posted this on both the financials and chit chat forums.