Here's another report with HP EliteBook 840.

== dmesg ==
DMI: Hewlett-Packard HP EliteBook 840 G1/198F, BIOS L71 Ver. 01.04 10/09/2013

== /proc/cpuinfo ==
model name	: Intel(R) Core(TM) i5-4300U CPU @ 1.90GHz

== lspci ==
00:02.0 VGA compatible controller: Intel Corporation Haswell-ULT Integrated Graphics Controller (rev 0b) (prog-if 00 [VGA controller])
	Subsystem: Hewlett-Packard Company Device 198f
	Flags: bus master, fast devsel, latency 0, IRQ 65
	Memory at d0000000 (64-bit, non-prefetchable) [size=4M]
	Memory at c0000000 (64-bit, prefetchable) [size=256M]
	I/O ports at 3000 [size=64]
	Expansion ROM at <unassigned> [disabled]
	Capabilities: <access denied>
	Kernel driver in use: i915


Steps taken:

1) Power on the computer without VGA attached
2) Log in to Unity

== xrandr ==
Screen 0: minimum 320 x 200, current 1600 x 900, maximum 32767 x 32767
eDP1 connected primary 1600x900+0+0 (normal left inverted right x axis y axis) 309mm x 174mm
   1600x900       60.0*+   40.0  
   1440x900       59.9  
   1360x768       59.8     60.0  
   1152x864       60.0  
   1024x768       60.0  
   800x600        60.3     56.2  
   640x480        59.9  
DP1 disconnected (normal left inverted right x axis y axis)
HDMI1 disconnected (normal left inverted right x axis y axis)
DP2 disconnected (normal left inverted right x axis y axis)
HDMI2 disconnected (normal left inverted right x axis y axis)
VIRTUAL1 disconnected (normal left inverted right x axis y axis)


3) Attach VGA cable
    - Desktop extends to the external display as it should

== syslog ==
colord: Automatic metadata add icc-ed869d76ba2f1549b15ab60e4327ba9b to xrandr-Lenovo Group Limited-VP-07998
colord: Device added: xrandr-Lenovo Group Limited-VP-07998

== xrandr ==
Screen 0: minimum 320 x 200, current 3520 x 1200, maximum 32767 x 32767
eDP1 connected primary 1600x900+0+0 (normal left inverted right x axis y axis) 309mm x 174mm
   1600x900       60.0*+   40.0  
   1440x900       59.9  
   1360x768       59.8     60.0  
   1152x864       60.0  
   1024x768       60.0  
   800x600        60.3     56.2  
   640x480        59.9  
DP1 disconnected (normal left inverted right x axis y axis)
HDMI1 disconnected (normal left inverted right x axis y axis)
DP2 connected 1920x1200+1600+0 (normal left inverted right x axis y axis) 518mm x 324mm
   1920x1200      60.0*+
   1920x1080      59.9  
   1600x1200      60.0  
   1680x1050      60.0  
   1280x1024      75.0     72.0     60.0  
   1440x900       75.0     59.9  
   1024x768       75.1     70.1     60.0  
   800x600        72.2     75.0     60.3  
   640x480        75.0     72.8     66.0     60.0  
   720x400        70.1  
HDMI2 disconnected (normal left inverted right x axis y axis)
VIRTUAL1 disconnected (normal left inverted right x axis y axis)


4) Detach VGA cable
    - Display setting switches to 1-display mode as it should (i.e. windows on external display are moved to the laptop display)
    - xrandr output is exactly the same as in step 2
    - Right after detaching these messages appear in syslog:

== syslog ==
kernel: [  172.974774] [drm:intel_dp_start_link_train] *ERROR* too many voltage retries, give up
# line above repeated 6 more times
kernel: [  173.010309] [drm:intel_dp_complete_link_train] *ERROR* failed to train DP, aborting
kernel: [  173.021468] [drm:intel_dp_start_link_train] *ERROR* too many voltage retries, give up
# line above repeated 6 more times
kernel: [  173.055319] [drm:intel_dp_complete_link_train] *ERROR* failed to train DP, aborting
colord: device removed: xrandr-Lenovo Group Limited-VP-0799


5) Attach VGA cable
    - External display is not recognized in System Settings -> Displays
    - xrandr output is still the same
    - Each time the cable is attached, this message appears in syslog 3 times:

== syslog ==
kernel: [  219.441432] [drm:intel_dp_aux_native_read] *ERROR* too many retries, giving up


Once the VGA cable has been detached the external display won't be recognized even after a soft reboot, UNLESS the cable is attached while rebooting. These steps apply only if the machine is fully powered off first...