The ornamental design for a sensor, as shown and described.
FIG. 1 is a perspective view of the sensor.
FIG. 2 is an elevation view of the front of the sensor.
FIG. 3 is an elevation view of the rear of the sensor.
FIG. 4 is an elevation view of the left side of the sensor.
FIG. 5 is an elevation view of the right side of the sensor.
FIG. 6 is a plan view of the bottom of the sensor; and,
FIG. 7 is a plan view of the top of the sensor.
The broken line showing of structural features is included for the purpose of illustrating non-claimed environmental drawings and forms no part of the claimed design.