I’ve done a lot of digging to find out whether an “s” in Spanish can ever come off sounding like a “z”. Everything I’m reading suggests that’s not the case. But I feel like I’ve heard certain words containing the “z” instead. For instance, when I hear native Spanish speakers say the last name Torres, it sounds like “Tor-ehz” and not “Tor-eys”. Similarly, when I hear the name Robles, I hear “Rob-lehz”. So, am I hearing things? How should these names be pronounced? Do words that end in “s” often come off sounding like a “z”? Or am I mistaken?
While it's difficult to know exactly how you perceive the sound of s in these words, it's entirely possible that it sounds (approximately) like [z] (the English z, a voiced alveolar fricative), or that is has acoustic qualities that register in your ear like [z].
In Spanish /s/ is a rather solitary phoneme, because Spanish doesn't distinguish between voiced and voiceless fricatives (as in the English pairs /f/ vs. /v/, /s/ vs. /z/, th as in thin and th as in then, or sh as in show and s as in vision). Since the Spanish /s/ doesn't have a voiced counterpart, it doesn't matter if you pronounce it [s] or [z]. Usually it's [s], but in certain environments some speakers may produce [z]. Native Spanish speakers won't usually hear the difference.
This happens independently of other variations in the pronunciation of /s/. Spanish has a lot of dialects and /s/ is one of the most variable sounds, so it shouldn't surprise you to find it pronounced differently from the standard that you've been taught. (For example, and in contrast to what you've heard, in my dialect it's very common to turn final /s/ into a barely audible [h] or to drop it altogether.)