You don't understand the syntax. This should work (I'm no expert on panels tho):

ON_T = mapon();
ON_Y = mapoff();

BMAP maze_map, <maze_s.bmp>;

PANEL maze_map_pan
{
POS_X 61;
POS_Y 265;
BMAP maze_map;
LAYER 3;
FLAGS REFRESH, OVERLAY;
}

FUNCTION mapon() {maze_map_pan.VISIBLE = on;}

FUNCTION mapoff() {maze_map_pan.VISIBLE = off;}