Three times Derek caught Stiles jerking off in the Jeep on the Preserve.

So the first time Stiles got Derek on his back, worked him open and pressed inside with the long exhale of a perfect fit, he wasn't expecting the sound. Sure when they exchange handjobs, or blowjobs, or any other kind of orgasm, Derek can get a little vocal – sometimes even demanding in a hesitant sort of way – but the deep moans, the sharp grunts, the cursing and the constant use of Stiles' name was not expected.