So is it likely this one merely escaped? I find it hard to believe someone who would own one of these would not be an enthusiast, and that enthusiasts wouldn't find another owner for a critically endangered species rather than merely drop it under a local bridge.
No it is extremely unlikely this is an "escape". This would be lucky to survive for a week in Europe. Almost certainly what happened is someone bought one and then realized they are too complicated to take care of and decided to dump it in a spot they thought looked pretty
Also there are 1,000 of these in the wild but there are over a million of them in captivity. You can get a typical morph for about $50.