They can show up in other ways. I noticed when my plain-color sheets suddenly developed polka-dots overnight. Turns out, it was my own blood. They'd engorge themselves, then I'd roll over and squish 'em.
Exterminator told me: "That's one of the worst we've seen. We'll send somebody out next weekend. Until then, keep sleeping there otherwise they'll go looking for food and get into your walls."
So trained staff may spot them but it is entirely likely that an infestation can go unnoticed until a costumer complains.