It doesn't always mean filler when someone leaves things loose. Sometimes it's just that they're wanting to try and keep their time low-key and not have plans at X, Y, Z that would be stressful to achieve.

Keeping you late for over an hour is a total douchebag move. A friend of mine did that to me once... after I spent 2 hours driving to the town where she lived (I was with my parents... which was stressful as it was). She ended up giving me like an hour... whereupon my mom checked her hair for head lice (her parents and my parents have been friends since well before either of us were born). Because she made plans with her bf... whom she lived with. I completed wasted that day. We aren't really friends anymore because that was how things always were and it wasn't fair to me.

If someone asks if I'm free, and doesn't solidify plans? Then I am free to do what I want... I'll usually message and go "Hey, I've got plans at X time, so let me know when you are free".