Remember that LK, aka "Deputy Dog", expense report master auditor, pariah, 6th down in the chain of the command from the President in his previous life as a reserve officer(or so he claimed to a bunch of kids in school on a career day) monitors these boards...
If your sick, your sick. If you get a call or a letter, tell them your sick, and that is it. Call the union. The company doesn't need to know why your sick, just that your unable to perform your duties or you were under a DR's care.
There has been a trend that if you have a sick day near one of the holidays or days listed in the contract that they will most likely ask for a letter. It has all but become SOP or so I'm told. You could call in sick for a trip on the 20th, an out and back, and not work till XMAS and still get a little because it is "adjacent" to a holiday.
Happy Holidays...oh by the way, two of my reserve trips had the same Flex Capt in the right seat on draft....we are overmanned, YEAH RIGHT!