I can think of one back in late 90s where a boat called Tawny Owl was stolen, it was recovered after I recalled meeting it in Lapworth locks. It was getting dark and we were at the last lock before the top lock and the boat with 2 males came down, they were trying to hide their faces which seemed odd at the time but I just dismissed it as some people see naturally unfriendly! The following day we were in the centre of Birmingham when we were asked if we'd seen it. We gave the info we had and the next day it was recovered heading for Hatton
Anglo Welsh lost one that took months to recover after it failed to return off a 2 week hire.
Vaguely remember Holly but again like the AW boat it was hired to be stolen
As for Que Sera Sera I suspected that it never existed and was a ghost boat i:e only existed on paper and a well organised insurance fraud, I may be wrong on this though