I have a set of points $(x, y) \in \mathbb{R}^2$ given by inequality: $|y| \leq \sin\left(\frac{1}{x}\right)$. And it's argued that this set isn't closed. I don't understand why.
As I understand, this set is points under the plot of $y = \sin\left(\frac{1}{x}\right),$ starting from $x = \frac{1}{\pi}$ and above the plot of $y = -\sin\left(\frac{1}{x}\right)$, starting from $x = \frac{1}{\pi}$. 
Help me, please, to find a limit (or boundary) point of this set which is not in it.