I have been doing a lot of research on this exact topic. It's a heated debate between UX and the development team who are in the process of implementing a new UI framework.
If you look at functionality on popular sites, you will find there is no consensus on tab behavior. Amazon does not move the user to the next field, Linkedin does, CNN does not. Frameworks behave differently as well; EXTJS does, Twitter bootstrap does not.
The DHTML Style Guide and W3C WAI-ARIA Draft are mostly consistent when it comes to keyboard behaviors, but unclear when it comes to autocomplete. However, if you look at how Tab should behave in a Combo Box (similar to autocomplete), the DHTML Style Guide states:
Tab moves focus into the edit field (Edit field to hold value of selected item in the dropdown list). A second Tab key selects the current item on the list, updates the edit field, closes the dropdown list, and moves focus to the next focusable item in the tab order.
If you're looking for consistency, that is how Tab should behave. If an item is selected in a dropdown (combo or autocomplete), that selection should be entered into the field and focus is set to the next item in the page tab order. If Tab were to keep focus in the same field, then the functionality of the Enter key is duplicated. With no other method to "select and move", the benefit of quickly filling out forms is diminished.
If there is no selection, then the dropdown should close, no selection entered into the field, and the focus is moved to the next focusable item in the page tab order.
The consistency of how certain keys behave in form fields is incredibly important. Keeping them consistent does so much to alleviate confusion and increase the confidence level in users of all experience levels.
- Side note: While Google does keep focus in the same field after Tabbing on a selection, notice the behavior on this Google API page for Maps. Here, once you've selected a location using the down arrow, tabbing off a selection does move the focus off the field (and onto the "All" radio button).