while there are a couple answered and unanswered issues similar to mine none of them quite fit my problem (or their solutions didn't work for me).
My issue: Using bumblebee I had no issues using Optimus on my EEE 1015pn (GT218 GPU) with 14.04. Now, with 16.04., neither bumblebee nor nvidia-prime are working correctly.
I've tried various drivers (340.101, 361, 367 ,370) and it doesn't work with any of those, while 340.101 appears to be the best working one, all OPTIMUS issues aside (it's the officially recommended one anyways).
I've narrowed down the issue so far, that apparently my GPU is no longer recognized as an available OPTIMUS-gpu.
sudo cat /proc/acpi/bbswitch
cat: /proc/acpi/bbswitch: No such file or directory
while trying to manually make load bbswitch from source I get:
insmod: ERROR: could not insert module bbswitch.ko: No such device
Both prime-select query and intel/nvidia work without errors, however after logging out and back in again, query always returns nvidia.
Given bumblebee's fickle nature I'd rather get nvidia-prime running first before getting bumblebee to run. So far installing bumblebee has only caused my NVIDIA driver to stop working altogether and bumblebeed is never loaded correctly as according to my syslog bumblebee can't find an OPTIMUS-GPU.