Microsoft Bing is testing a new annotation in the autocomplete search results to communicate to searchers that they can tap the tab key to finish the autocomplete without clicking on the term. Here is a screenshot of this that I can replicate - no you cannot click on the word "tab" but using tab on your keyboard does do the work.
This was spotted by Khushal Bherwani and he posted this on Twitter.
Honestly, I think most searchers would find that annotation very confusing and not know what to do with it. But I guess it is worth testing...
Forum discussion at Twitter.