The highest version of Windows Phone that the average Joe has access to is 8.0.10328.78. Considering that the first three numbers on the mysterious versions are 8.10, what Myles could have discovered is Microsoft doing some internal testing of Windows Phone 8.1.
Expected to be coming on Windows Phone 8.1 is a notifications center along with improvements to multi-tasking capabilities, and a screen rotation lock. It seems that Microsoft will push out the GDR3 update prior to the Windows Phone 8.1 update and that some features, like the screen rotation lock, require GDR3 to be installed first. This means that you shouldn't expect Windows Phone 8.1 to start rolling out until sometime in 2014.
Back in June, mobile ad network AdDuplex spotted Windows Phone 8.1 being tested on a trio of devices including the Nokia P4301 test device, the Nokia Lumia 920 and the HTC Windows Phone 8X.
source: WPDang via WPCentral