I remember when my dad worked for a major car manufacturer back in the 1970s. He was a member of the local United Auto Workers chapter, too. Every once in a while, he would call his supervisor at the plant and claim to be sick. He told me that the union guys called it "blue flu", but I thought it was a real disease at the time. He'd stay at home for a few days, then go back to work.
I have to believe the managers at the plant knew it was a planned sick-out, since the odds of 30 people calling in sick on the same day were pretty high. Blue flu may have cost the company some productivity, but it wasn't something the managers could prove was a strike. Every worker there was entitled to a number of sick days, anyway.