458 Coolant reservoir?

That is an overflow tank I believe.

 

Should be about 1/2 full when hot. Check and replace the screw on cap. They leak a lot with age and will allow coolant to overflow easily. Prob can top it off to mid level for now with whatever coolant is in it and see how it behaves.(Don't mix different types of coolants) Better yet, If possible do a complete coolant flush, and replace the cap. That should solve any uncertainties.

 

The proper level is up to the seam on the tank when cold. I like to go slightly more, and as it heats up, it will purge any excess (there is a hose where it comes out right in front of the right rear tire). Once it purges out the excess (sometimes it may take several months to create the parameters that make the level rise, so don't panic if it dumps fluid a second time at a later date) it will usually wind up about 1/2" above the seam of the tank.

 

I forgot to add, this is the way I have seen dealers do it as well.

 

If you find yourself topping off coolant and its been a while since a fluid change then good idea to get the old stuff out. Its probably pretty weak and your engine and systems will benefit. Most owners overlook the health of radiators and such.