In a casual conversation, one person didn't know he unwittingly revealed he doesn't like whatever, ice cream. The other person says... 你无意之中流露出来 which translates to roughly to "You let it slip." I think. So does 你无意流露出来 mean the same thing? The only difference I see compared to the above is the above explicitly references to the subject, ice cream, they're talking about?
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.