From manual:

TR receiver:

E$; Mark the square as an object entrance (yellow trigger square)
$ = 0 means yellow trigger.
$ =1 means no yellow trigger (1).

But check works too: E?y1

ZVSE

!#TM99:S1/999/1/255;
!?TM99;

!!TR30/84/0:E?y1;
!!HE-1:P?y2/?y3/?y4;
!!FU&y2=30/y3=84/y4=0:E;**if there is an hero on that square, quit function
!!FU999&y1=0:P;
!?FU999;
!!UN:O30/84/0;

or

!!UN&y1=0: etc

Fast way to avoid WM or resource pop-up on that square.