Autocomplete search is a tricky thing to track. The underlying logic is that when the user starts feeding characters into a search form, the search suggests results based on a limited input. If the user is not satisfied with the results, they can continue adding characters to the search, thus increasing the accuracy. Often there's also the option to revert to a regular search with what they've already written. Tracking this logic in tools like Google Analytics is difficult, because there's really no way to know if the search was successful.
Analyzing what people write in your site search field is pretty much one of the smartest things you can do for your website tracking. If certain terms pop up over and over again in internal search reports, it means that your site is not providing the answers people are looking for, meaning you have an excellent opportunity to provide supply for the demand! However, not all site search applications are trackable out-of-the-box.