If the 2.8 mm screw securing the antenna cable to the Mac mini's logic board “disappears” and cannot be located after the motherboard is extracted, the screw has propably settled in below the speaker. There is a neodymium magnet in the speaker and it will attract the screw. Loosen the speaker to collect the screw - do not try to push the screw out with a tool as you might do damage to the motherboard. The whole operation of retracting the screw is less than 2 minutes.