I'm not sure I've got this down to a repeatable pattern yet, but the behavior is certainly "odd". I've got a ThinkPad X250 with an associated docking station. I have a Dell P2715Q (4K) monitor plugged into a display port on the docking station. The monitor seems to be detected OK and work fine upon first being plugged in, and it certainly works OK if it's plugged in from boot.

Things start to awry when the monitor is disconnected (either by removing the computer from the docking station, or by removing the monitor plug). At that point, "xrandr" starts taking a long time and, irrespective of the actual monitors plugged in, reports a phantom monitor on the port where the now-disappeared monitor was:

DP-2-2 connected 1024x768+1920+0 (normal left inverted right x axis y axis) 0mm x 0mm
   1024x768      60.00* 
   800x600       60.32    56.25  
   848x480       60.00  
   640x480       59.94  

At this point, no further monitor plugging/unplugging seems to have any effect. This is on Debian's kernel 4.9rc5. I'll test with 4.6 next and see what I get. 

Related (but not certainly identical) bugs include bug 98211.

Debian bug template: https://gist.github.com/inducer/4a382ef00efc10fe1104f701267fe999 (kernel/software versions, logs, etc.)