Google recently published the official schedule for their I/O developer conference, giving attendees and developers around the world an opportunity to pour over the upcoming events. One interesting detail is the reference to Android M in the schedule.
Android M will be the eventual successor to Android L (Lollipop) which was announced at last year's Google I/O, followed by a subsequent developer preview release. The official name and over-the-air updates didn't arrive until Fall.
The reference to Android M can be found under the Android for Work event, scheduled for 2:30 PM PST on the 28th. The presence of a What's New in Android session (1:00 PM) is also a tip-off. Google usually uses takes this opportunity to discuss what's debuting in the freshly unveiled version of Android.
source - Google | AndroidPolice
[UPDATE] The Android for Work session has already been removed from the schedule, indicating that the company was not quite ready to share the existence of Android M.