Got an overflow bottle on there? (Y'know that thing that some dinosaurs among us just don't get.) You may be just measuring what is expelled by expansion on the first run after a top-off.
Nowadays, radiators (esp cross-flow) rarely have an airspace at the top to allow for
expansion at running temp. The o'flow container is meant to siphon back to the radiator on cool-down and maintain o'all volume. No reservoir means air is drawn back in, and leads one to suspect a leak somewhere. (
Which radiator you have doesn't make any diff)
I'd check more often to see if the loss was sudden (as I said
might be) or gradual over time. There will be evaporation, too, depending on radiator condition, airflow, outside temp, clock hours, etc ... W/o an overflow container your radiator will never be 'full' when 'cold'.
btw, I had GF who kept topping up her 'catch can' and was puzzled as to why it lost AF by the next morning. System was always full under the radiator cap. Her 'morning addition' was blown out when the car got warm. The o'flow tank went from too full, to empty as coolant was drawn back into the system overnight and air replaced it.
I often recommend that folks just look at an o'flow container and see if the level varies from time to time, warm to cold, etc. If it does (vs static & leaving a scum line) it's a sign that the system isn't leaking, thus is siphoning coolant back into the radiator from the reservoir as intended. To do so translates to 'no leak'.
