You show her the spot.

You finish your beer fast so you can head to the bathroom before you go. You could swear you felt a hand on your ass a few times while you were in there. You must be going crazy. You head out to find the waitress ready to go.
"I'm Amy, in case you were wondering," she says as you head out together. You lead her in the direction of the place. "So you really like big dicks, huh?"
"Yea," you admit, but add hastily, "just not when they are attached to guys. I'm not gay...it's just an ass thing."
"It's cool, I didn't say you were. I like anal so I don't see why guys wouldn't. I like to put on my fat strapon and pretend that I can fuck myself. Of course I can't, but I can do you if you want. It's a big fat twelve incher. Would you like that?" She grabs your butt.
"Hey!" You are surprised at her forwardness.
"Nice ass," she giggles. "So do you want my fat dildo in your ass? Do you want your ass to be invaded by a big piece of rubber?" She is rubbing your ass now. "I can tell by that tent growing in your pants that you would."
"Yea, that would be nice," you tell her. "It's just a bit further," you note as you get near the spot you were scammed. You arrive and show her what you did to get your little toy. She busies herself with the same process you went through. She sure takes a long time with the description. You glance about until you feel something wet touch your hand. You look but nothing is there. Your hand feels a little slippery as you rub it.
"I see...we are going to be having more fun than I thought..." You look and see nobody again.
"Done!"