Mark,
I agree with Opie. I too am unfamiliar with your model, however it looks like the cap has come apart. Just get a new OEM cap from any of the online suppliers or your local dealer. They are inexpensive. Once you have it on hand, carefully compare the new cap to the old cap and its components, then remove the faulty cap/part from the reservoir.
As far as engine temperature not raising until you are 4 miles down the road.....On our F models 2013-18, the temperature gauge actually measures oil temperature, not coolant temp. It does take a while for that gauge to budge, even here in Florida. Double check what yours is measuring, coolant or oil.
Lastly, you could always get
Bimmerlink and get all of the above and much more displayed on your phone.
Let us know how it works out for you.
L