FIG. 1 is a perspective view of one embodiment of an in-wall humidity sensor illustrating aspects of the new ornamental design; FIG. 2 is a front view thereof; FIG. 3 is a left side view thereof; FIG. 4 is a right side view thereof; FIG. 5 is a top view thereof; FIG. 6 is a bottom view thereof; and, FIG. 7 is a back view thereof. The environmental structure shown with broken lines in the drawings forms no part of the claimed design.