I'm not really sure if this is regular practice with MSN (Bing) or not. Since a few weeks back, they've been spidering my website without a proper User Agent (meaning their UA does NOT say anything about bingbot. It's just a regular browser UA). This happens from three different IP ranges. Yes, I've checked thoroughly and the IP rangers are registered under MSN.
I doubt this is manual review, since it doesn't look at all as normal human behavior.
Did they officially stop declaring their bots in User Agent strings?