We claim the ornamental design for a sensor for a merchandise security device, as shown and described.
FIG. 1 is a perspective view of a sensor for a merchandise security device showing our design;
FIG. 2 is a rear view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The dash-dash broken lines illustrate portions of the sensor for a merchandise security device. The dash-dot broken lines represent the bounds of the claim. The broken lines form no part of the claimed design.