According to the gospel accounts, Christ spent most of his time out of Jerusalem, in the countryside, travelling around. He had certainly been to Jerusalem before – remember his clearing the moneylenders out of the Temple forecourt? – but he didn’t stay there for long periods. It wasn’t safe.
On the occasion of his arrest, he had only been in Jerusalem for four or five days. His other stays may have been similarly brief.
And, remember, if you hadn’t seen Jesus in the flesh, you really had no idea what he looked like. There were no pictures – a verbal description was the best you would have to go on. And Jerusalem was a big city; there would have been lots of people in Jerusalem who had never seen Jesus, or certainly who hadn’t seen him closely enough to be sure of recognising him again in the dark.
The High Priest’s soldiers would certainly have wanted an informer to tell them where Jesus could be found to be arrested, and would probably have wanted one to to identify him for arrest. Bear in mind that, while the Gospels present Jesus as allowing himself to be arrested without either hiding or resisting, the High Priest’s soldiers couldn’t have known that he would do that.
Of course the symbolism of the Judas story is immense and, even if the story is not a fabrication, it may well have been tailored somewhat. But, simply looking at the gospels as a factual narrative, the story is not implausible.