$modx->user->get('username'); // returns the admin account name instead of (Anonymous)
As long as you are logged into the manager and use the same browser to view the front-end, it inherits the manager user info/permissions. This is so you can view unpublished documents, see the site when it is offline, etc. You can use a different browser (or incognito window in Chrome) to view the front-end without this behavior.
I can’t reproduce this on my system with Firefox on any version of MODX Revo.
BTW, I noticed in FF that you can’t do a "View Source" on unpublished docs in the front end even though you can see them -- you get the source of the error page. I’m not sure about other browsers.
It turned out that something was indeed messed up with my Firefox -- caching was disabled, so there was no cache to read from! Specifically, the following preferences in about:config had been set to false: browser.cache.disk.enable and browser.cache.memory.enable. Once I set them back to true (which is of course the default), then I was able to verify with HTTPFox that the view-source requests were reading from the cache instead of making a new request.