yes in fact it is stealing because you can return the bitcoin to the address from which you have receive accidentally and if you are not doing so then you are real doing wrong.
yes if you know the person or the address from where you get the fund then it is really stealing but if yo do not know the person then it is not stealing but you have to not use it tell some time and if the owner not fount then you can use it.
yes i also agree with you that if you know about the person or address from where you get the balance ans still you are not returning it to him then it is really stealing, but if you do not know abut the address then it is not stealing.