Someone from Netflix writes to me:
The post office doesn't scan any DVDs for us. The envelopes come to us from the post office in standard mail cages. The envelopes are opened and inspected (currently by hand) by the nearest Netflix hub starting extremely early in the morning. After inspection, they're scanned by a computer. It's not until the DVD is actually scanned that it's marked as returned.
Netflix has spent quite a bit of time hacking the USPS, as it were. They've found they have a much higher customer satisfaction rate, as well as being easier to get new customers, if one-way transit time is only 1 day. It's hard to achieve in most places, but in high density areas like the SF Bay Area it's very cost-effective.
The same correspondent references this article on related information, and more.