[Rob175]

Let the system cool overnight and then check the tank in the rear. It could be the tank is leaking like on my 21 year old tank was before it was replaced.
Pull out the trunk liner/carpet and anything else that blocks your access/view of the metal trunk floor.
(in my case there was a couple of small "pools' of anti-freeze collecting there).
Many of the aging tanks develop "spider cracks". leak a little coolant into the trunk, and expand from the pressure in the tank when hot.
It can be difficult to detect if there is little or no "pooling" of coolant in the trunk, but can still cause the systems temp to rise.

Suggest you wrap some paper towel around the tank sides and bottom and go for a drive to heat up the system.
After a good drive and fully hot, check the paper town for moisture/coolant stains.
If not totally dry you may need to replace the plastic tank, like I did.
Also, it could be the thermostat is bad, and/or the radiator is blocked with leaves or debris.
(I vacuum out the twin radiators as best I can with "reach" tools especially in the fall and leaves are falling.

So, check the tank for spider cracks, clean the radiators of any debris, and finally consider a new Thermostat.
......or a final thought....it's possible you have air in the system that needs to be purged.