My app engine site redirects links to the site index page to the same URL with this query string appended: ?_sm_au_=iFVtD2FW4H4wSHS6.
It only seems to happen in Chrome. Other browsers do not get redirected.
I'm using the default App Engine Java framework. I'm not sure if the query string is coming from App Engine or from Chrome. I have been unable to find any documentation on _sm_au_.
That query string is usually related to Google AdWords cookies. It is used for retargeting ads.
If you're not explicitly using AdWords in your project, you might have an ad-injecting Chrome extension installed. You may double check the latter by accessing your app/website from an incognito window.