A property of the single layer potential & compatibility conditions in 2D?

So I'm reading a journal and I have a surface $\displaystyle S$ of a domain $\displaystyle D,$ these areas are extended with a $\displaystyle t$ variable, and another surface $\displaystyle \Gamma$ of a domain $\displaystyle E$ which contains the domain $\displaystyle D$ entirely, also extended with a time variable. I also have a dunemerable, dense set of points $\displaystyle \{x_k,t_i\}$ on $\displaystyle \Gamma\times(0,T].$ $\displaystyle F(x,t,y,\tau)$ represents the fundamental solution of the heat equation, $\displaystyle f$ an arbitrary continuous function and the integral

$\displaystyle \int_{0}^{t_i} \int_{S} f(y,\tau) F(x_k,t_i,y,\tau) \,dS d\tau = 0.$

Then it introduces the potential of a simple layer

$\displaystyle V(x,t) = \int_{0}^{t} \int_{S} f(y,\tau) F(x,t,y,\tau) \,dS d\tau$

from the above equation we know that $\displaystyle V(x_k,t_i)=0,$ and because $\displaystyle V$ is continuous and is equal to zero for a denumerable, everywhere dense set of points $\displaystyle V(x,t) = 0$ for $\displaystyle (x,t)\in\Gamma\times(0,T].$ The journal then states that $\displaystyle V(x,t)=0$ in the whole external region outside of $\displaystyle E$ (Basically $\displaystyle V=0$ in the infinite space $\displaystyle E^c,$) (with no explaination why) I was just wondering why this was the case that $\displaystyle V=0$ outside $\displaystyle E,$ or do I need to give more details.

Also does anyone know the compatability conditions for the heat equation in two dimensions or somewhere where I can find them on the internet, (its so I can state that these allow there to be a unique solution to the heat equation).

Any help would be greatly appreciated. Thanks!

(Thanks mush for the tips)