I was trying to see if the arrow graphic that appears in Google Music Trends was being stored in some common Google directory (where other services may also use it from), or if it was specific to Google Trends.

I ended up finding that everything was stored in the "http://www.google.com/trends/music/resources/" directory. But all of the graphics, javascripts, stylesheets, etc. had a large number at the start of the filename:

3492087938-favicon.ico
3831612697-logo_mtrends.gif
3859418412-up.gif
28643867-down.gif
1254494484-mtrends.css
1105625842-mtrends.js

Any idea what the significance of the number could be? I don't think I've seen such non-standard naming on any other Google services, so I found it quite interesting.

Could it be some kind of versioning so that they can create new versions without worrying about them being cached in the user's browsers?

That's a thought, though I wonder why they would use such large numbers.

Maybe it's not a version number but a random number, with the effect Tony mentions. Something like a "soft" GUID. Or a Google developer hacked in the number manually :)