According to the Wikipedia article on the defence of Ciudad Rodrigo the siege lasted from 26th April to 10th July 1810. However, a book on the military exploits of Juan Martín Díez makes no mention of the city or the siege, noting that he was busy elsewhere...
The Empecinado continued under the orders of that general [Bassecourt], until at the earnest entreaty of the Junta of Guadalaxara General Bassecourt allowed him to return into that province at the end of April 1810.
However [Juan Martin] so well planned his operations and marches as to be able to fall upon [the enemy] frequently, and make them pay dearly for their plunder. Upon one of these occasions the action he had with them in Solanillos, on the 15th of May, deserves particular notice, for although the enemy's corps was considerably stronger than Martin's, he hung upon it for a distance of four leagues...
On the 18th [May] the enemy, reinforced to the number of one thousand five hundred, came out of Guadalaxara in pursuit of the Empecinado, who retired to the banks of the Tagus.
In the course of the month of June, [Juan Martin] had a number of sharp actions with the enemy immediately close around Madrid, so that the destruction of the Empecinado became an object of serious consideration...
The enemy took possession of [Brihucga and Siguenza] on the 29th of June, leaving in Brihucga one thousand two hundred men and two pieces of cannon. On the 30th Martin had already commenced operations against them, he was assisted by Don Francisco Palafox, who had five hundred infantry under his orders. He beat them out of the suburbs and forced them to shut themselves up within the walls of the city, where he was obliged to leave them, having no artillery with him.
His cavalry had now increased to the number of four hundred, his infantry to one thousand one hundred; with this force on the 16th of July he again attacked this garrison, and at the same time so to intimidate the three garrisons of Brihucga, Siguenza, and the temporary one of Torelaguna as to shut them all up in their respective posts for many days.
So there appears to be little time for him to have been caught in Ciudad Rodrigo.
The military exploits, etc. etc. of don Juan Martin Diez: The Empecinado, 1823, pg60-61