According to an official statement from Google, links from other local sites are used to geo-locate your business:

“Other signals can give us hints. This could be from local addresses & phone numbers on the pages, use of local language and currency, links from other local sites, and/or the use of Google’s Local Business Center (where available).” source

How Google identifies the location of the website or business is another story. It’s probably a mix of IPs, physical address if any, telephone numbers and so on.

If user intent requires it your website may show up in Web Search, not only on Local or Maps.