When something is sourceless, it means it has no origin or cannot be traced back to where it came from.
Being sourceless can be mysterious or confusing because you don't know where it came from.
Examples:
The rumor about the treasure was sourceless.
The strange sound in the forest seemed sourceless.
The sourceless painting appeared overnight.
The sourceless smell of cookies made everyone hungry.
The sourceless light in the sky puzzled the scientists.
The sourceless laughter echoed through the empty room.