The Vulgate, Syriac, and Ethiopic render "συνετηρει" as "kept in custody," but that is not the only translation of the word. Thayer gives another definition of "συντηρέω" as "to keep within one’s self, keep in mind (a thing, lest it be forgotten)" (Thayer's Greek Definitions). In Luke 2:19 "συνετηρει" is translated "treasured" (NASB, ESV, NIV) in the sense of "saw the importance of":
"But Mary treasured all these things, pondering them in her heart." (Luke 2:19, NASB)
The point in Luke 2:19 is that Mary recognized the importance of and noted the things spoken by the shepherds. This is close to the meaning of "observed."
In Mark 6:20, we read:
"Herod feared John, knowing that he was a just man and an holy, and observed him; and when he heard him, he did many things, and heard him gladly."
Herod recognized John as a holy man and heard him gladly. The context thus favours the translation of "συνετηρει" as "saw the importance of" or "observed." Herod did not just leave John in prison. Herod saw and heard John. This is best described by "συνετηρει" being translated "observed."
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