Si quelqu'un se demandait, comme moi, où était passé le surveillance station, voici la réponse du support technique :
Dear customer ,
Thank you for your support inquiry .
The Surveillance Station APP has been removed from APP Center of HS-251 temporarily .
Please download it from :
http://download.qnap.com/QPKG/SurveillanceStation_5.1.1.4.3_20170601_x86_64.zip , unzip this .zip to a .qpkg file . Then install it in APP Center of this HS-251 manually ( see attached picture ) for a try . Let me know the result . Thank you