I just want to say one thing & i hope it doesn't sound assumptive or judgmental. But IME with every ISTP I know... If a girl seems "really nice" to you, she's not interested... she's friggen' ravenous.
I have actually heard that "I don't know if she likes me... she wore makeup... keeps asking me things... talks about her SO" thing so many times & usually these girls turn out to be absolutely floored for them. You guys have a way of being so laid back but non disclosed that you make it almost impossible for someone to be "all over you" when they want to be. They throw hints as subtle as they think yours 'might' be.
And you guys are so nonjudgmental that if you even have a doubt she might want more than friendship you are probably 'trying' not to make a judgment by discrediting what would be obvious to intuition.
Also, I don't know how common the ISTP type is. But to me you guys are so perplexing that you seem exotic & foreign, while at the same time obviously stable & self sustained. Perfect fantasy lover "get me away from my terrible relationship" profile.