I don't know what an 'id' is - it usually refers to a componenet of the subconscious mind, as per Freud. In that sense, they are never seen.
Perhaps you mean IDs, such as identificaton tags. Anyhow the question can be answered without that knowledge.
"IDs are not to be seen" could mean "You must keep you ID hidden".
"IDs were not to be seen" could mean "We couldn't see any IDs anywhere."
Student or Learner