What could explain this, besides the supernatural?

Anonymous

My dad, when he was younger, and his two friends were riding their motorbikes one night in the countryside. They saw a girl with long hair covering her face and a white dress, walking towards her. They asked where she was going late at night alone, and my dad reached his hand out. He swears that his hand went through her. They turned around and she was gone. This was the middle of nowhere, with no place or building to hide. They swear it was a ghost.

What could explain this, besides the supernatural?
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