Before Safari build 60 was released I saw all build 58, 59 and a bunch of earlier builds in my logs, mostly from apple.com IPs.
$ grep Safari * | perl -pe 's/.*(AppleWeb.*)/$1/' | sort -u | tail -1
AppleWebKit/62 (like Gecko) Safari/62"
Amusingly it's not from 17.* IP addresses (Apple), or even IPs that looks like they are from California.
ThinkSecret posted a report about build 62 saying that it has tabs (yay). Maybe it leaked?
ObSafari: Neato trick. Shut down Safari and run from a terminal window:
defaults write com.apple.Safari IncludeDebugMenu 1
Watch a useless Debug menu show up. You'll like it. You'll feel geekier.