Yeah the geo locations feature is something that's set as public. It's not like you put a secret tracker on her or something. 
Just tell her "look, you ghosting me and soending time with your ex makes me uncomfortable. So when I looked at your geo location and saw that you were at his house it gave me a feeling that you are still sleeping with him. If you are, just tell me so we can both move on." 

Be firm and direct. No sense on beating around the bush