Miguel Sánchez de Plasencia was present at the 1540 muster as a horseman in Luna y Arellano's company. Because there were two other Miguel Sanchezes on the muster it is possible that he added his place of birth to distinguish himself from the others. Also, another member of this company, Pedro Nieto, was a native of Plasencia (Cáceres province) and that may connect the two.
1540-1550:
1540, Feb.: Muster in Flint & Flint, Documents, Document 12 [two horses, elk hide jacket, native arms, Luna y Arellano's company]
“Disposal of the Juan Jiménez Estate,” in Flint & Flint, Documents, Document 27 [Coafor, March 18, 1542, Sánchez de Plasencia serves as witness]
Other info: ?Piferrer, Nobiliario, 4:153, #1758 [many people used the patronyms like Sancho, especially from lineages from Castile, Aragon and Navarre; progenitor via reliable sources was Rodrigo Sánchez; escudo - quartered, 1st of gules and a gold castle with 2 keys high up of silver, which is de Alcázar; the 2nd is silver and a banda of sinople with gold dragons linguados de gules, which is Sánchez; 3rd is García; and the fourth of gold and una faja of blue and en el gefe, encima de la faja un leon de gules rapante, which is Ponce de Leon]