This is most likely a plot hole. The best logical explanation is that she heard their voices and most likely caught glimpses of them from around and under the desk. That or she saw through the bullet holes in the desk.
Another possible explanation is that Tammy saw them in the hallway, since the Albino said she left her cigarette burning--"She does that," said Eddie Lomax. But since we never see them actually meeting, it would be reasonable to assume they saw each other before somehow, even on a different day before Tammy hid under the desk. Yes, otherwise, it is a flagrant plot hole.
Tammy says clearly to Mitch that two guys looked for her the next days and the guy who informed her about that described them.