According to a report from PhoneArena, a new Android L build was recently spotted on Google's Chromium tracker. It supposedly shows off some UI tweaks which include altered Navigation Keys and Status Bar Icons.
In case you haven't heard of it, Chromium tracker is a bug submission tool for Google products. This latest submission was the LMP LRW87D build running on a Nexus 5 and at first glance didn't appear too different than current Android. However, after closer inspection, you can definitely make out some slightly tweaked elements, especially in the he software navigation keys and the status bar icons.
Above is a combined image to compare the differences between current Android KitKat and these Android L changes.