I have a Dell 2007WFP (Analog) external monitor which doesn't get picked up on the Display panel. I'm running a ThinkPad W520 with switchable graphics; the graphics card is a NVIDIA Quadro 2000M. I ran the Additional drivers which didn't detect anything required to install.
I have the W520 too (with Quadro 1000M) and two external monitor (via docking station) I think you must search for the Nividia Optimus problems.
For a 'quick solution' you can change the display configuration in the BIOS settings (F1) from OPTIMUS to integrated (=intel chip) or the other setting for the Nvidia card (my setting) .