They got sucked in due to the negative pressure created by the tsunamis around the globe.
Or, because of the overcrowding of cellphone frequencies they had to add a new range, which happens to coincide with the impedance of these plates. This causes an inductedd current that melts them.
Perhaps the troll ate them.
The TSA took them for screening. They can’t get the x-ray to go through it, so they’re being outsourced to China in meanwhile.
The CIA noticed that rain bounces off these things. That made them very suspicious. An unidentified black car came by and snatched ’em.
Really it’s all the fault of pigeon feeding. That makes the ground sticky. A car tire gets sticky and then rides off with these covers unknowingly.