Where does Delana note that? She says:

True, she would have obeyed that hypothetical beggar if he had the proper signs and said the right things, but Halima’s hands had identified her as Black Ajah, which she clearly was not. Perhaps the woman thought that was the only way to make Delana acknowledge her, and perhaps she wanted to show off her forbidden knowledge as well. Delana did not like this Halima.

There is no mention of "all BA knowing these signs" only that Halima had forbidden knowledge. Even the signs the beggar would give her may be completely different things than the BA signals.

I agree that Talene gave up every scrap she knew, but they make no mention of a sign that would help them identify a woman who was BA. In fact they state that they were at a dead end because the women who's names they knew were not in the Tower. No comment that they could have tried using some secret signals or leave messages in Talene's hidden spots and watch ... Alviarin would have likely moved those around every time anyway.

Speaking of Alviarin, your own arguments about her paranoia support that only a select few would know these signs. An easily identifiable sign would throw the entire Heart system out the window and would give Alviarin FITS