Patrick was being paid to take you on a date so your sister Bianca could date, and she was desperate for it.

So Cameron and she were in your room after "useful" things for Patrick to convince you. "we won't find anything" says Cameron

"of course we will" says Bianca and time has passed and they find several things like concert tickets, posters, books and etc, but Bianca found something even more useful...

"Aha! Black panties" she says excitedly

"what does that mean?" Cameron says excitedly.

"Just tell Patrick that she had these bands, these books and that she has black panties he will understand" then Cameron agrees and later meets Patrick, he says about everything they found in his room until he finally says

“by the way Bianca said that Kat has black panties and that you would understand." says Cameron.

Patrick doesn't answer, he just smiles and goes out looking for you.