I get the same from my Defender and can’t find the source/leak anywhere.
Its been going on for over 2 years now and I only need to add coolant every 6 months 🙄
After you've parked, as your walking around the rig, does it smell like radiator? I had this happen, but nothing was leaking... Then I was stranded on Vail Pass. Steam, steam and more steam. What was it????