I recently received an email informing me that "an unknown computer" had attempted to log in to one of my accounts. Now, some of these emails are really phishing attempts by Nigerian scammers.
I did some investigating and noticed there were what appeared to be log-ins as well as attempted log-ins that had been made targeting my Yahoo! and Google accounts. No one was able to penetrate the levels of security on my G+ or Google, BUT it looks like someone was "inside" my Yahoo! - they didn't do any damage or change anything as far as I can tell.
Just to be safe, I went to the public library, logged on to Yahoo! and changed my password. When I got back home, I deleted all of the cookies in my computer that I didn't recognize and then I downloaded the free malwarebytes program and ran it on my home computer. It found some questionable stuff, and I allowed it to scrub all of it from the machine - which now runs FASTER than before!
Word to the wise: Just to be safe, rather than sorry: remove any checkmark that says anything like "Keep me logged in" on any portals/websites you regularly visit. I know it can be a pain to keep logging in, but look at this way - you're protecting your online presence (and you're not likely to forget your password)! Surf on!