After my visit here today, I can witness that this GC is not present in GC4DAR5. It is obvious that this GeoCoin has been lost between September 12th to October 11th. Geocacher "happybiker", who found the cache on Sept 12th as the first after "Mofos", retrieved "a GC". It could have been your GC, or somebody from next finders could retrieved it and forgot to log it. However, there is still a hope for you to find it - you should contact "happybiker" and other cachers and ask them, whether they saw your GC in the chest. The box (chest) is located in the museum and cannot be muggled; there is only one way of loosing trackables: when a cacher take it with him and not to log it.
Unless your investigation is successful, I ask you, Cpt. Feldi, to mark your coin as "missing". Currently, it can be wherever, but absolutely not here. (One of the GC and TB cemeteries, where GC can "rest in peace" forever, is e.g. cache GC1A8K2.) Thank you in advance.