I agree, there's really nothing that can be done but talking. That's the only way to find out what's going on in her head or in his. She is probably the one to start with, since she's the one with whom something may be going on. You can do it in person or via email (email might be easiest), but just initiate a conversation!
You could start with something like "Hey, there's something that's been on my mind and I'd like to have a conversation about it. I really liked living with you and your husband, and I like you a great deal, but I was confused by a few things. For instance, the way you called me your husband and calling me your life partner. In addition, a mutual friend has said that it seems like we're always flirting. Before I can consider moving back in I need to know where we stand. Am I just misinterpreting? Are you interested in a deeper relationship than just friendship? If so, how would your husband react? For my part, I am very happy being your friend and do not need anything more than that, but would be willing to consider another arrangement IF it's something you're looking for and it wouldn't mess up your relationship with your husband. Let me know what you're thinking.