The Unknown Mexican is mentioned in the Radios in the Escalation Zombies Map Call of the Dead
Nobody knows who this person is or was, hence the name.
He/She was killed during testing at the Siberian facility where Call of the Dead is set, as Richtofen states that "His Spleen is on the floor and he is not moving". A monkey then ran of with the spleen, Richtofen chased it, nobody REALLY knows what happened to it because the radio ended.
Point is the subject died...
Some rumours around the community suggest that it could be Danny Trejo, this isn't possible as he is still alive in the events of Call of the Dead.
The other speculation is that this subject may be the blank portrait that is found in Kino der Toten, there is no way of proving or disproving this, so it may be correct.