Firemen warnings work different from Police warnings. You get a warning, if a random number between 0 and the number of sleepers is nonzero, or a random number between 0 and 9 is zero (if you have no sleepers you are warned by the "word in the underground").
// Sleeper Firemen can warn you of an impending raid
int firemensleepercount=0;
for(int pl=0;pl<pool.size();pl++)
{
if(pool[pl]->flag & CREATUREFLAG_SLEEPER&&
pool[pl]->type==CREATURE_FIREFIGHTER)
{
firemensleepercount++;
}
}
if(LCSrandom(firemensleepercount+1)>0||!LCSrandom(10)) {
erase();
set_color(COLOR_WHITE,COLOR_BLACK,1);
move(8,1);
if(firemensleepercount)
{
addstr("A sleeper Fireman has informed you that ", gamelog);
} else
{
addstr("Word in the underground is that ", gamelog);
}
move(9,1);
addstr("the Firemen are planning to burn ", gamelog);
addstr(location[l]->name, gamelog);
addstr(".", gamelog);
gamelog.nextMessage();
refresh();
getch();
}
As you see, there is no deterministic way to get a warning. It is better to be equiped to fight them off than to rely on warnings.
Note, that they only raid locations with a printing press and liberals present. If nobody is at the location of the press they can't raid you, but you can still publish secret files. Just daily writing needs someone present. An unoccupied printing press is also a good way to publish CIA files without becoming wanted for treason.