A story on Macworld UK today notes that Apple has warned Mac users to check the back of their MacBook for signs of a small piece of plastic covering the vent. “First fruit of these investigations: the company published a technical note yesterday, in which it warned that some MacBooks ‘may have left the factory with a thin piece of clear plastic covering the rear vent.’ According to Apple, this thin plastic slice is used in the factory to prevent dust from getting into the machine. ‘If your MacBook has the plastic still over the vent, simply remove and discard it,’ advises Apple. The company has also published a second document for notebook owners who think their machines are running too hot, in which it explains that a certain amount of heat is perfectly normal,” notes the report.