The ornamental design for a cap for marking and detecting an undergroundutility, as shown and described.
FIG. 1 is a top, side and front perspective view of a cap for marking and detecting an underground utility.
FIG. 2 is a top view thereof;
FIG. 3 is a front view thereof, the left side, right side, and rear views being identical;
FIG. 4 is a bottom view thereof; and,
FIG. 5 is another top, side and front perspective view.
The broken lines defining of a central circle on the top of the design and of a stake in FIG. 5 depict environmental subject matter that forms no part of the claimed design.