A question on Quora entitled "What happens if I put someone else's Google Analytics code on my website?" triggered a train of thought and investigation that made me discover things about how Google Analytics treats its Hostname Dimension, and because of that how you can discover which other domains are using your Google Analytics tracking code.
The answer to the Quora question is that anyone can include your analytics code on their own website. They won't get any information from it, but they could confuse you.
If they put your code on their website, you will see their traffic as if it is yours, on top of your own websites data.
luckily, Google Analytics provides a Dimension (their term) so you can see the Hostname (domain) that causes any traffic. So if someone does, for some strange reason, want to add your Google Analytics code to their domain, you will known who they are. And can hunt them down!
Google Analytics also lets you create custom reports, so I created one that showed all the Hostnames that are causing visitors on the websites I manage, and I was surprised...
I didn't see malicious competitors causing most of the dodgy traffic, I saw major websites!
The domains I picked up that triggered the most traffic tended to be translation websites (Google), caching websites (Google), social bookmarking websites (ShareThis) and URL shorteners (TinyUrl). This got me intrigued.
I could not work out why many of the Hostnames were showing up. There seemed to be no link between them and me!
After analysing these websites I noticed something in common, many of them use frames (spit) to embed peoples websites. You know the thing, A top bar to keep you on their website while they display yours.
This means your website is displayed, but within a frame of someone else's website. And your website triggers your tracking code.
So I think the discovery here is, if your website is placed in a frame of another website, you get to track that via the Hostname Dimension in Google Analytics. I.e. You can see if other websites are displaying your website within theirs. Google Analytics must be tracking the Hostname of the top most frame.
You may be surprised how much traffic is due to this. On my small website 0.5% of traffic was given a different Hostname. For my international clients I'm seen figures up to 3% due to the translation websites.
It may be worth filtering out Hostnames in Google Analytics, or at least looking at them to see if anything fishy is going on.
Now to analysing all those other Hostnames to see why they are triggering traffic on my website!