When a user is typing in an address on the address search box on the locator they will be shown a list of suggestions based on what address Google thinks they are asking for. It requires 3 characters or more to be entered before it shows a suggestion.
Enabling This Feature
In MySLP go to Advanced Options > Experience > Search > Functionality > Search Form Address Processing
In WPSLP go to Store Locator Plus > Experience > Search > Functionality > Search Form Address Processing
Check the Show Address Guess box.