There's no requirement to disclose during an interview, but most people I know depended on their time in the reserves/active duty pretty heavily on their resume

There's no requirement to disclose during an interview, but most people I know depended on their time in the reserves/active duty pretty heavily on their resume, so I'd guess it was pretty rare for an employer to not know or at least suspect. Also they have have leave to train for 2 weeks per year, so the employer would need to know at some point. As for chances of being deployed, in the last decade it was probably a good bit higher than the chance that a reproductive age woman gets pregnant for a given time period. They can also end up deployed for longer than most countries paid maternity leave lasts.