Microsoft has been reportedly working on four new smartphones, including two flagship devices and two mid-range smartphones. Previous reports had hinted at a fall 2015 release date for Microsoft's new flagships, but had failed to provide any specific details. Microsoft's flagships and mid-range devices are expected to be launched together with Windows 10 for phones. Since Microsoft is likely to push Windows 10 for phones to vendors by the end of Q2, these smartphones could be unveiled in early Q3.

Now, AdDuplex, which serves cross-promotional ads on Windows 8 and Windows Phone apps, claims in its Phone Statistics Report to have detected recent activity from a new Microsoft Lumia handset with a 5.7-inch, 1080p display and running Windows 10 exclusively.

AdDuplex, which is exclusively allowed to run ads on Windows Phone ecosystem, has a pretty good track record of sniffing out unannounced Windows Phones. In February, the company had detected several mid-range and low-end Lumia phones, which were officially confirmed by Microsoft in the following month. According to AdDuplex's Windows Phone Statistics Report for April 2015, which analyzed monthly statistics collected from 5,429 Windows Phone apps running AdDuplex SDK v.2, it has spotted a phablet codenamed RM-1085 with a 5.7-inch Full HD (1920 x 1080) screen and running Windows 10 for mobile.

AdDuplex has not confirmed if the phone it spotted is the new Microsoft flagship, but it is likely to be a successor to the Lumia 930 or Lumia 1520, as Qualcomm has claimed that the 2015 Windows flagship phone will be powered by its Snapdragon 810 SoC. Microsoft has said that it will not release a new flagship Lumia phone before the launch of Windows 10 this summer. This means that the device could be released by around June this year, the same month that Windows 10 will be officially released.

Microsoft's BUILD conference begins on April 29, and rumors suggest that since the Nokia Lumia 930 – the earlier Lumia flagship – had been launched around this time last year, the Redmond giant could provide a glimpse of a new Windows 10 smartphone at the event.