How long had it been since you checked it? That’s about a half a quart, which isn’t much if you hadn’t check it in many months, especially if your coolant system, hoses, etc. are OE. Make a note of the date you added, then check it weekly.

On the other hand, if it had only been a week between your last check of the coolant level, then you are likely loosing it somewhere. Not many places it can go….on the ground, into the motor via a bad head gasket/out the exhaust via the heads, or you could have a heater core/plumbing leak. There is a small chance too that your transmission heat exchanger has failed (if you have one), but I don’t think this is too common.

Do you smell coolant under the hood/wheel wells, etc. when the car is warm? If so then it could just be leaking into something hot and evaporating.

__________________
Current
Original owner 2002 E53 X5 4.4i to 4.6i swap
2026 G80 M3 6 spd
2025 G06 X5 50e
Former
1972 Audi Fox
1986 Saab 900S
1996 BMW Z3
1998 BMW E36 M3 Sedan
2004 BMW E46 M3
2006 Audi A3 Quatro
1993 Mopar 318 Jeep Grand Cherokee
2015 V6 Jeep Grand Cherokee