I don't know exactly what Coinbase does, but online services often move "your" funds around. I talk based on my experience at Poloniex.
Basically it's not fully your wallet, it's their wallet and they only monitor the deposits to that address and then tell "that's your money".
When you withdraw, it's again another address(es) you get the money from. But it's okay, it's their optimization and it usually works fine. If you don't like it, use your own wallet (and you should know that online services are not really the safest and greatest wallets).
Of course, there's a chance that you've sent to wrong wallet or you are facing a hack/stealing. However, you should contact their customer support and check with them.
I had multiple times in my life funds deposited at online services which didn't show up, but after filling a ticket the account was credited. Of course, you have to send the proof (account ID, tx ID) to help them check.