I created a coinbase account and a new wallet that only had one address "A" associated with it.
I then gave that address to a friend and had them send me some amount "B" of bitcoin.
Shortly thereafter my coinbase wallet was listed as having the correct amount "B" of funds and the address "A" associated with my coinbase wallet shows up in the blockchain as having the correct amount "B" sent to it. However, my address "A" immediately showed up in the next block spending the funds, and the address "A" is listed as spent (e.g., on blockchain.info shows up as "spent"). There are a large number of subsequent transactions spending the funds to a bunch of addresses that I have never seen before.
Does anyone know why this is happening? Is this some attempt at coinbase to perform some mixing and try to anonymize transactions? Or does coinbase send the funds to a "cold storage" or something and just pretend like they are "in" the address listed in my coinbase wallet... very curious...