Even though Jacob had no idea what was going on nor did he get to see enough for longer than a fraction of a moment, he was able to tell that this familiar-looking girl, who had practically thrown herself all over this certain guy, was the same one he had seen with Marcellus a couple of times.
"Are you seeing this, Jacob?"
"Yes, Bianca. Seems like a troublesome commotion."
That petite figure, her shoulder-length brown hair, and a somewhat conspicuous presence - indeed, they're all pointing to the person he's suspecting. However, there are also her strange girlishness, that squeaky, high-pitched tone, and the impression that she seems to be caught up in some love triangle - he couldn't remember her acting like this nor did he expect that she had such a side all along.