As a manager at 3 small individual doctors offices. I don't ask what's wrong if someone doesn't offer. It's the 3rd day for us. I actually think this was lifted from the Dept of Labor. Once you call in the 3rd consecutive day, then we can ask for a doctor's note.
However, I did recently let go a young women who called in 5 times in a one month period (her first month of work I might add). Anyhow, she was never out for more than 2 days in a row). However, she appealed her denial of unemployment, so I had to get a bit more involved w/ the Dept of Labor. They did tell me we could require a doctor's note for each "sick day" an employee takes. Also, we could prohibt someone from using paid sick time for sick kids or spouse's (the girls excuse was her son was sick each time). However, we couldn't fire them if they got an excuse from there kids or spouses doctor for sick days. But there was a limit on that too. The laws are defined, but very confusing. If we have someone abusing sick time, we usually start to look for another reason to let them go. Usually if they aren't there often, then their work is suffering and that will be the avenue we'll begin to take.