If its head gasket I would expect the loss to be consistent. If you open the rocker cover cap / oil filling point and it looks like there is Mayo in there, then it could be a sign the head gasket is failing.

I’m afraid that your partner has a coolant addiction and you need to get them checked into rehab.

On a serious note, perhaps sitting in traffic is causing the temp to increase and causing a leak. This would make me think perhaps the radiator fan isn’t working properly?