Is there a difference between 你无意之中流露出来 and 你无意流露出来?
The first one is straightforward - it means showing one's intent unwittingly. The second one is a bit tricky to interpret. It does mean the same thing in the examples you quoted. However, "无意之中" and "无意" are not the same. For example, I can say "他无意上学" (he doesn't intend to go to school) but it is wrong to say that "他无意之中上学". Note that "他" makes no attempt to hide his intent here. "上学" (going to school) simply doesn't cross his mind.   

Also, "之中" means "in the midst of doing someting". Grammatically, it's present continuous or past continuous.