Thermostat

Morning Folks, another question regarding neighbours

2004 Hippo TD4. I had topped up his coolant a few months ago but noticed it had dropped a little, looked around the bay this morning with no obvious signs of leak, any advice on what else to check?

Temp seemed ok also, had it running for a few shopping runs yesterday & temp stayed around this, see pic.

Coolant loss can be attributed to cracks in the expansion tank around the cap, which is a common issue.
Coolant can also be lost through a number of O ring seals, which are at key joints in the cooling system, the common one's being the steel bypass to thermostat connection. Or the top hose outlet from the head, although this one normally looses coolant pretty quickly.