I've been having a lot of issues with my i5 Surface Pro 3 with the Intel HD4400 graphics card. The issue happens most on Windows 10 TP, but I've also seen it some while running Windows 8.1. This has happened across 4 different Surface Pro 3 devices so it's not nothing specific to just one device.
When I have the Surface Pro 3 docked and it entered a connected-standby state after being inactive awhile most of the times when I come back and move the mouse / keyboard the external monitors won't wake up. I know the device is powered on because I can see the Surface Pro 3 keyboard light up but the monitors are still in a sleep state.
I'm during the Intel HD Graphic Driver 4/28/2015 (10.18.15.4204) device ID 8086&0A16.