Magento – CSS Caching.. Where is it hiding

I'm getting a bit frustrated with caching as I can't figure out where the CSS files are being cached, and I'm hoping someone can help me out.

Here's the situation.

• I am amending the RWD theme to create a custom design
• when I make changes to the phtml or xml files, flushing cache allows me to see the changes
• when I change to make changes to the CSS files, I can see them being compiled by compass, and I can see my changes on the server in the newly created styles.css file
• No matter which cache I clear, I don't see the changes on the site until an arbitrary amount of time later. Some Times 5 minutes, sometimes 30 minutes plus
• If I browse to the CSS file location from a browser, the changes are not there, yet FTPing to the site, I see the changes.

Here are the various methods I have tried to clear cache (I have done all of these together with no success):

• Through admin – flush cache, flush cache storage, flush CSS/JS, clear full page cache
• Deleting contents of var/cache
• Deleting contents of media/css and media/css_secure
• Flushing browser cache
• Turned off merging of CSS files in admin

I have even tried a browser on a separate PC that has never loaded the site before, and the changes are not visible.

Am I doing something silly? I hope so, but at present, I just can't complete my design as I can't test the changes I am making to the CSS in live without a massive time overhead.

Any help is greatly appreciated.

Thanks.