Garsantos analysis incoming
The first scene we see them in together in season 2, Garcia is paged to assess a patient and asks for the p presentation.
Santos looks up at her pitifully and does a motion that reads (to me) as her thinking "oh god. I have to talk to her like everything is normal after the disagreement we just had" or whatever went on beyond the camera. (I also think Garcia has been acting pissed off while in Santos' presence, but I feel like that acting is "blink and you'd miss it" since Garcia is just kind of has this intense surgeon aura (and surgery is just as swamped as the ER). But I digress.)
At this point we've already established that Garcia HAS a toothbrush at Santos' place and is willing to talk about their situationship out loud to coworkers. That's not casual behavior. That's a woman who knows what she wants.
We've also seen Garcia think of Santos fondly when reminded of her in the scene where Ogilvie goes to do his first intubation and the camerawork cuts to Santos.
The next real moment we have with them is when Santos asks Garcia about their plans for the 4th. Garcia says she might need a raincheck. Wuh-oh. What do friends and lovers do together on the 4th of july? Watch fireworks together. This scene is Santos establishing that they've crossed this boundary of fuckbuddies, but she's done or said something to piss Garcia off.
The next tidbit we get is when Garcia gets called for another consult and slides in next to Santos with the quip "you know you can call me just to say hi sometimes". Our girl is petty š© she wants more and we know Santos wants more. Santos is just too emotionally constipated.
So when we get the scene were Garcia says "we're just keeping it casual, right?" She's one thousand percent throwing Santos' words right back at her. The wording. The tone. The LOOK she gives Santos. Garcia wants more. Santos wants more but doesn't think she deserves it, or is scared of commitment, or whatever.
And Garcia knows it. So Garcia is showing her what Casual partners means. It means drawing a hard line in the sand where no one is happy. And she's hoping it wakes Santos up a bit.