The ornamental design for the security cap for plug lock, as shown and described.
FIG. 1 is an isometric view of the present invention from a front end;
FIG. 2 is a side view of the present invention, all sides being identical; and,
FIG. 3 is a front end of the present invention.
The broken line showing in FIGS. 1-2 is for illustrative purposes only and forms no part of the claimed design, the rear end has no ornamental features.