Defendant arrested for evading. Had a pocket knife on him. Officer puts knife in his pocket during arrest. Finds it in his pocket after defendant has been transported to the jail. Back at the office, officer opens knife and finds cocaine residue on blade. Uses it to link defendant to some dope found at the scene. Warrant needed?

A warrant should not be required. Police may still perform searches of arrestees for weapons for officer safety and the prevention of escape. Which, by the way, your defendant had just attempted with his evading. Since the knife was on the arrestee, it is still in his immediate area of control.

Since the knife was legally obtained, inspecting it later and finding residue of controlled substances in plain view shouldn't be a problem. Recent case law has changed the vehicle search exceptions and the digital media (cell phone) search exceptions, but I don't think any of the recent cases change anything here.