Exactly. Assuming they took a cold shower in the morning, sounds like this would wake them up in an aggressive way, probably raise their adrenaline level which made them less likely to call in sick. Once at work and levels wore off, you were already at work so you'll push through. Once you need to write things up, you have same day of sick days but more days when you were at work.

If there was a long term (day long) effect in reducing "feeling sick" there should be less sick days as people who are borderline sick start to feel fine and don't report being sick. But if your feeling is temporary, let's say 2 hours then you can get to work and after the fact report that you were sick.