Any chance your partner uses the heater and you don't? Many cars have a heater core bypass valve that only allows coolant into the heater core when the heat is on in the cabin.

Are you sure it's coolant? I know older focus had issues with the wiper liquid hose cracking.

When coolant heats up, it can evaporate of there's a way out. Do you have any residue on the inside of your hood?

If you're experience a LOT of coolant loss it could be escaping into the engine through a falling head gasket. In this case you wouldn't see any residue on the inside of your hood.

God speed.

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?

Does your partner blast the air conditioning?

Either way you should get a coolant system refresh and pressure test asap

Not a car expert, just a guy that learns how to do something when it needs to be done...

I don't know how relevant the "why" is... you've got a leak, and the easiest first step is to inspect to see if you can figure out where the leak is originating from. If you can figure it out, then fix that. If not, the next easiest might be to just replace the coolant lines. If it's still leaking after that, it could be either the radiator or a gasket.

Good luck!

Yes, heat can play a role.

I’d recommend going to your local parts store (or probably Amazon) and getting UV dye specifically for the cooling system. You dump it in, run the car for a bit, then start looking with a black light (you’ll probably need to buy one of those too). The ones we use flash a bright yellow/green where the leak is originating from.

If that’s not something you’re comfortable with, call your trusted mechanic and see if they can do it.

If you’re losing a considerable amount of coolant, don’t let it go, you could be making a relatively simple repair significantly more expensive, or possibly even killing the engine, if it’s as bad as it sounds.

Also, has the vehicle ever been overheated before, or is it getting hot when she drives it?