A couple of months ago during Ramadan, I was outside before Fajr and the adhan for Fajr came on yet I didn't see the white horizontal thread. I googled it to see if someone experienced the same and it turns out that sometimes time tables used by mosques may be wrong. Perhaps I'm mistaken and something was overshadowing the white thread. Still, I think it's better to be on the safe side. Here is a fatwa regarding the matter: