It seems that Microsoft has started rolling out Office 2016 to some of its Office 365 business subscribers. Some users with business subscriptions are reporting that they have been prompted to download the latest version of Microsoft's productivity suite.
Microsoft had previously announced that Office 2016 would arrive for certain business users this month. Users with an Office 365 ProPlus, Enterprise E4, or Enterprise E4 plan were on the schedule to see Office 2016 at some point in February, so if you're on one of those plans, you may want to check for an update.
This could be a phased rollout, as some are reporting that they can only update Office on one PC, despite being signed into their Office 365 account on multiple machines. Office 2016 rolled out for consumers in September 2015.
Thanks to Nicholas for the tip!