Owners of select Android One smartphones have reportedly started receiving an official update that brings Android 6.0.1 Marshmallow to their devices. As you may know, Android One handsets are mainly sold in developing markets (by local companies). Thus, we're not surprised to find out that the first smartphone to get Android 6.0.1 is the Cherry Mobile G1 - a device that's available in the Philippines.
It's not clear exactly what Android 6.0.1 is bringing, but it's definitely a minor update, likely including bug fixes. What we can say is that this isn't related to Google's monthly security updates, since the November security patch has already been released - without changing the Android version number (all recent Nexus devices, including the new Nexus 5X and Nexus 6P, still run Android 6.0).
HTC ROM developer @LlabTooFeR has it that, after 6.0.1, Google is planning to roll out Android 6.1 Marshmallow, though this is expected to happen starting next year. Needless to say, Android 6.1 should be a more important update compared to 6.0.1.
Sources: Reddit, @LlabTooFeR