You can see in the screenshot that the results are hidden behind a "Show results" dropdown. There's no risk of people spotting your subject lines by peering over your shoulder at search results.
Check that picture out. Immediately, people can tell that you have email from a certain sender about the topic that you have just searched, even before you click "Search results".
To just get all the comments out of the way:
@adgar2: Senders and subject are available: Subject is the search query, sender is seen right there in the screenshot. Looking over the comment, I probably communicated that people could see the body of the e-mail. That's my mistake, I meant to say that people could see senders and subjects.
@Karunamon: It's real obvious when I'm opening my e-mail that people can see my e-mail. I wouldn't expect that when just searching Google.
All in all, as I said logging out is a perfectly viable option, but the slip up at the wrong time (e.g. getting some spam that passed your filter, searching "ass" while on your way to finishing typing assassin's creed or whatnot, and having "Gmail results: messages from xxx..., Jane Doe,...and 2 others" show up).
