Silvery - 5:15 pm on Aug 24, 2009 (gmt 0)

Typically, the plus sign appears on a listing when the page/site is about a particular business which Google has matched up with their business listings within Google Maps.

Google Maps, other search engines and other business directory sites get their data from a great many sources, chief among which are what's called "data aggregators" - companies which compile business data and sell it to other companies such as yellow pages and search engines. It's not all that unusual for an error such as yours to get absorbed by a data aggregator and then replicated out across many sites.

I suggest trying to get the error corrected with the data aggregators, InfoUSA and Acxiom. Also, attempt to claim your company's listing with major yellow pages sites as well as with Google:

[google.com...]

Finally, you might also post your question in the Google Maps Help Forum:

[google.com...]