The shadow mice are doing their job extremely well. The first group of half-elves is easily noticed by Sam due to their movement. But there is a bigger threat hiding far away from the place that is watching him with a piece of special equipment.
He is too far away from Sam's spiritual sense, energy vision, and even with the zooming ability combined with his eye technique is not enough to get a clear image of that person. They are deep inside the forest kilometers away.
In fact, they were only noticed because shadow mice made a long trip around the forest on Sam's order so that he could use the whole forest as his operating base and set traps to defend himself.
They noticed this person who is completely covered in black clothes, almost similar to Sam's dark-cloaked attire and from the descriptions of the mice that saw him, he is using a device similar to a small telescope to look at Sam.