"[Y]ou have the right to ask your employees questions about their leave of absence so as to determine whether FMLA applies. So, asking them why they were out and whether they've been out for this reason before is acceptable.... Asking them to confirm that they actually took leave for the reason provided? This is where it gets a bit tricky.... [T]his kind of question [carries a] risk that it tends to treat differently those who are returning from FMLA leave versus those who are returning from other forms of leave, thus causing the employer to be susceptible to an FMLA interference claim."