A WebmasterWorld thread has one webmaster complaining that Microsoft is hiding the user agent for the spider, BingBot.
The webmaster said:
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?
So either Microsoft has a weird bug, someone is spoofing the crawler in a weird way or Microsoft is doing this on purpose?
I should note, I did not check my log files to verify this on my sites. So I am going with "zerillos" post and with no back up evidence.
Forum discussion at WebmasterWorld.