A number of early 2013 Retina MacBook Pro owners are reporting fan speed issues, according to plenty of reports around the web including on Apple's own discussion forums. Specifically the issue is about the laptop's fans kicking on when there is no need to. Users report only stressing the laptop minimally, such as just having a web browser open with a few tabs, will cause the fans to spin for minutes at a time. Users also report that the laptop isn't even warm while the fans spin.
Geek.com suggests the issue could be due to a fan management software glitch in MacBook Pros using new SanDisk SSDs. Apple switched from Samsung SSDs to SanDisk SSDs in some units of the latest MacBook Pro with Retina display. There's been no official comment from Apple at this time, but some users have been able to get their MacBooks replaced when reporting the issue. Other users have been told it's a normal occurrence. If a fix does come, many suggest that it would be through a software update and that no replacement of parts would be required.