The wording of Shield of Faith suggests that it can be used to reveal a known but invisible enemy:
A shimmering field appears and surrounds a creature of your choice within range (60 feet), granting it a +2 bonus to AC for the duration.
Unlike many spells, there’s no requirement for the caster to see the target.
A Jeremy Crawford citation indicates that there needs to be a clear path from the caster to the target.
A fairly common "boss fight" scenario is that the heroes enter the boss’ lair, the boss delivers an ominous speech, everyone rolls initiative, and on the boss’ first move they cast Invisibility (or Greater Invisibility). Would casting "Shield of Faith" on the boss be a cheap counter? (The boss getting an AC boost, of course, is just hilarious).