When using the T420 / T520 on the docking station with external monitors connected via DP and VGA ports. The boot up screen is not showing even with the laptop lid close. Boot up screen would only show on laptop screen.
This is a limitation of Optimus capable systems.
The reason is that the integrated graphics processing unit (iGPU) is providing the signal to internal LCD. The discrete GPU is linked to the DVI, DP ports (attached screenshot), so that is why the only setting available for Optimus is the "ThinkPad LCD". We have tested all possible settings and we can confirm, that the "Boot Display Device" is always offering the correct available options. Integrated and Optimus settings allows the integrated chipset to drive the internal LCD. Discrete offers the whole variety like analog VGA, digital on dock etc.
If it is important to have the ThinkPad logo displayed at boot in your particular configuration, the BIOS will need to be set to discrete video.
BIOS Setting; Config -> Display; set Boot Display Device to Analog (VGA), Graphics Device to Discrete Graphics.