The Codex Syriacus is not the same as the Codex Sinaiticus which was removed from the Monastry by one von Tischendorf. There is a fascinating history of the Codex Sinaiticus with ensuing discussion of its immense significance.

Exerpt from Al Ahram by Jill Kamil
The holy fathers of St Catherine's exercise much secrecy and reserve regarding their heritage, especially their sacred manuscripts. This is largely owing to their unfortunate experience with Konstantin von Tischendorf, a German scholar from the vicinity of Leipzig. He took a precious codex (sinaiticus), the oldest translation of the Bible into any language, to old St Petersburg and gave the monks a handwritten note saying that he was taking the work on loan in order to copy it and promising to return it undamaged. The monks counted on the return of the precious codex, but they never saw it again.

The great significance of the Codex Syriacus or the Syrian codex, which is now of international fame, is that it is the only known copy of the Greek New Testament in its original uncial script. It was discovered in 1892 and the text is a palimpsest -- which is to say, a text partly erased so that the parchment on which it was written could be used again, as indeed, it was. In this case, the underlying fifth-century text is now so faded as to be virtually invisible.
Von Tischendorf presented the Syrian codex (synaiticus)to then Czar of Russia Alexander II. After Tischendorf's death in 1917, however, the Russian revolution in the same year resulted in financial problems for Russia which caused the precious bible, consisting of 346 folia and a small fragment, to be sold to the British Museum for the then enormous sum of GBP100,000. Little wonder that the monks were subsequently reluctant to let anyone gain access to their library. They have been suspicious of scholars who wish to carry out research in their archives ever since -- even if they hold the highest credentials.

See also http://www.saintcatherinefoundation.org/codex-sinaiticus-syriacus (right click open in new tab)
1892 Translation https://archive.org/details/translationoffou00lewiiala A translation of the four Gospels, from the Syriac of the Sinaitic palimpsest by Lewis, Agnes Smith, 1843-1926