There's a difference between not trusting his intentions and openly calling him out on it in the most confrontational way possible. Surely there's some safe middle-ground in there somewhere. Anyway, if detective Stern had had other intentions, he probably wouldn't have snapped back the way he did, since that could've ruined his chances at... whatever it was he was trying to accomplish.

"Well, ideally I won't see him again," Leo points out. Detroit is a big city, the chances of bumping into him aren't that big. Unless Leo goes and gets himself arrested again. But he generally only does that when he's high, and he has no plans of getting high any time soon if he can help it.