The ornamental design for a walldetector, as shown and described.
FIG. 1 is a perspective view showing a wall detector.
FIG. 2 is another perspective view showing a wall detector.
FIG. 3 is a front view of a wall detector.
FIG. 4 is a rear view of a wall detector.
FIG. 5 is a top view of a wall detector.
FIG. 6 is a bottom view of a wall detector.
FIG. 7 is a side view of a wall detector; and,
FIG. 8 is the opposite side view of a wall detector.
The broken line showing of structural features is included for the purpose of illustrating non-claimed subject matter and forms no part of the claimed design.