Yes, strictly speaking a salt is not a secret, and would generally be stored with the data you are salting.
If you change the semantics and make the salt a secret that is stored separately, it does make this difficult to attack, but the advice I was given is it would be better to use hmac, which is already designed to work this way based on storing a secret.
Okay, that's what I thought. Under my current use-case, I think what I'm doing is quite adequate. But, your post is very relevant to what I think I'm going to need to do at some point soon, so thank you!
If you change the semantics and make the salt a secret that is stored separately, it does make this difficult to attack, but the advice I was given is it would be better to use hmac, which is already designed to work this way based on storing a secret.