It's not disinterested body language (this mean she caught a glance at you and thought "eww"). It's more hiding that they are into you. Like you caught a slight tenth of a second them looking at you then they look away fast. So that you kind of doubt yourself for a second (hmm was she looking at me or is that just my imagination?). In this situation you need to follow your instincts (if they are telling you. I think she was. So I'm gonna approach her). This is her not trying to appear easy. Whereas if she didn't care about being perceived as easy, when you looked over she would meet your eyes with a smile. Both those situations you have to approach. It's VERY likely that she likes you. I have had both these outcomes lead to lays.
More what I was talking about, for example, last weekend, I said to these two girls that had just being approached by loser guys, "what did they do wrong? I thought they were doing good? I was trying to take notes" or something self amusing like that. One girl said "idk haha" then kind of body language like she was leaving. The other stayed put, smiling and said, "they were boring" like encouraging me to keep talking to her. This is her making it easy for me. Subcommucations saying, "I'm interested in you."
Lastly this is the latest girl I'm working on, who I have a date with tommorow after work
So I say the days I can see her. And she tells me the whole situation. Allowing me to make the judgement call that will work best in our favor (I don't want to deal with her cokblock friends so I opted for daytime meet).