It's definitely suspect, the way she's acting, but Derek's the last person to be throwing stones in that particular glass house, so he doesn't ask, just nods and lets her take her sweet time retrieving the papers he's picked up. "I can do that," he agrees. It isn't until the uniforms pass and she visibly relaxes that Derek realizes they were the source of her discomfort. That's interesting... Whatever joke he thought was underlying her comment before about him turning her in, seems like it wasn't actually a joke at all.

"No, not really," he agrees, looking confused and curious. "Why?"