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I am sure that this doesn't happen all the time though, perhaps someone who understands browsers better would be able to explain why sometimes a referal string is sent and sometimes it is not.
Referrer information usually comes from either the HTTP_REFERER or HTTP_FROM headers - depending which one is being used by your report, it is possible to forge quite easily. For example, my logs occasionally show referrers such as "++---------------------".
If there is definitely no link on the other page, my best guess would be that one of the headers has been modified by a server-side script running on your competitor's site.
Why they would want to do that is another matter altogether!
A referrer is not returned if the visitor types your URL into their address bar.
Perhaps a referrer could be returned if there was a bug with the browser. I just see this too often, and from obscure sites, for it to always be someone faking the referer.
Joshie what was the browser in your referral string ?
Some browsers will return a referrer if a url is typed. We get a few dozen referrers here a day from places like NewsHub, Cnet, Wired, ZdNet. We've collectively tried to figure out what it is that triggers a referer like that. It has something to do with the interaction between the back button and caching under IE 4-5. I think IE6 has it fixed.
As mark mentioned, there are also browsers with known referrer bugs. Currently Opera 5 has a pretty serious one in that urls clicked into new windows doesn't always return the referrer, and it can also return the previous root domain instead of the page you visited.
It's been my experience, you can throw out 10-25% of your referrs as bogus depending on the traffic origin.
Which looks like a fairly standard IE5
We have even seen referrals from images (.jpg, .gif - I know!!!:)) in the past which, naturally, we've put down to buggy browsers.
I guess, as brett said, you just can't rely on your logs.
The only explanation I could come up with was someone with a buggy browser typing in his URL after viewing my robots.txt file.