I don't think any 'light boat' out of Wilderland ever crossed the borders of Lorien. Thus this traffic originated from the south of Lorien, or at least it reached the river there, while the east-west traffic crossed the river fare more in the north.
It was unknown were Thorongil came from. And Thorongil did conceal that by purpose. Thus I believe even if he would have been asked directly about Imladris he would not have shown greater knowledge than what was common in upper class gondorian society (like Denethor).
Respectfuly
Findegil
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