When a stranger's Apple ID appears on your Mac in conjunction with an update for an app that you have never installed, you have a pirated app on your Mac. The pirated app is not the app requesting an update. This pirated app has been cracked with the MAS receipt from the app calling for an update, often Angry Birds or Twitter, acquired by the person whose Apple ID you see.
You may not have realized that it was a pirated app when you installed it. You may have believed that it was a trail version or a "free" version.
All MAS apps have a MAS receipt in their app bundle. Check any app acquired from the MAS. Right click on the app's icon in your Apps folder and choose Show package contents. In the Contents folder is a _MASReceipt folder and in that folder is the coded MAS receipt. It has the Apple ID of the MAS account that bought/acquired the app from the MAS. It is that receipt that the MAS uses to alert you that an app has an update. There would not be a notice of an update with someone else's Apple ID showing in the MAS on your Mac, if there was not an app with a receipt from this person's account somewhere on your Mac.
Download the free app FindAnyFile (FAF);
http://apps.tempel.org/FindAnyFile/
This app is easier to use and seems to be more powerful than Spotlight. Open FAF and in the box type _MASReceipt and press Find. It should come up with a list of every app on the Mac with a MAS receipt. Compare the apps with receipts in FAF's list with your purchased apps list in the Mac App Store. The odd app out should be the pirated app.
