Bouvier's Law Dictionary, Revised 6th Ed (1856):

SIGNA, civil law. Those species of indicia (q.v.) which come more
immediately under the cognizance of the senses, such as stains of blood on
the person of one accused of murder, indications of terror at being charged
with the offence, and the like.
     2. Signa, although not to be rejected as instruments of evidence,
cannot always be relied upon as conclusive evidence, for they are frequently
explained away; in the instance mentioned the blood may have been that of a
beast, and expressions of terror have been frequently manifested by innocent
persons who did not possess much firmness. See Best on Pres. 13, n. f.;
Denisart, h.v.