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[JJoe]

The sign-in link looks something like

Code:

http://www.google.com/accounts/ServiceLogin?uilel=3&service=youtube&passive=true&continue=http%3A%2F%2Fwww.youtube.com%2Fsignin%3Faction_handle_signin%3Dtrue%26nomobiletemp%3D1%26hl%3Den_US%26next%3D%252F&hl=en_US&ltmpl=sso

The ASCII character ":" can be represented as "%3A" in BASE16 aka hexadecimal aka 'hex'.
Some more:
/ is %2F
? is %3F
= is %3D
& is %26
% is %25

"&" is an html entity that translates to "&"

After the link is unescaped and the entity translated ( to make it easier to read )

Code:

http://www.google.com/accounts/ServiceLogin?uilel=3&service=youtube&passive=true&continue=http://www.youtube.com/signin?action_handle_signin=true&nomobiletemp=1&hl=en_US&next=%2F&hl=en_US&ltmpl=sso

Clicking the link would take you to google to sign-in and then return you to youtube, I think.

So, the link contains some escaped (BASE16) ASCII characters and may tell google where you were and where you wish to go. It's a "Base 16-Tracking Link".

The set tries to provide a direct path to the final destination. The original link is in the flyover just in case.

The user-agent is recent but not the latest.
I think the current stable's user-agent is

Code:

Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.2.13) Gecko/20101203 Firefox/3.6.13

HTH

Have fun