Watch when Cell is destroying the islands. The two remaining Androids are hiding on an island. A rock flies by and 18 is about to shoot it, but 16 says that Cell would sense her blast. But Androids don't have power levels, so he couldn't have sensed the blast. This is also evident when Cell is perfect and 16 sneaks up behind him and grabs him. Piccolo says that Cell couldn't detect him because he's an android. So that means that if 18 fired a blast, Cell could not sense it.