Reverse IP geo-targeting is one of the most commonly used techniques by trademark infringing affiliates. They hide themselves from occaisional monitoring by showing their ads to every geography except the geography where they believe the brand-holder is monitoring from.
For example, Gap's corporate headquarters is located in San Francisco. A Gap affiliate could target their ads to every city except San Francisco, or every state except for California. At the time of this writing, searching for 'Gap Coupons' in CA would turn up no ads. However, the same search conducted from our offices in Seattle turned up this ad:
As you can see from the bottom of the ad, it has been targeted to users in WA. I'd expect to find similar ads in 49 other states.
Identifying Reverse IP Geo-targeting
There are a bunch of ways that you can identify this. The most commonly suggested method is to utilize a proxy server to browse with a different IP Address. I find it easiest to use a service such as Google Translate, or Yahoo Babelfish.
If you are utilizing an automated monitoring system, you should make sure to ask how the monitoring system circumvents reverse ip-geotargeting. Look for systems that conduct searches in multiple locations. Of course, if you are using PoachMark to monitor your search advertising, we do this automatically.