Vjylku

Zone of Truth contains the following clause:

You know whether each creature succeeds or fails on its saving throw.

In a scenario of checking someone's loyalty, any countermeasure that allows you to always succeed your save against this spell will alert the caster that you have succeeded the save. They will keep you in the area of the spell until you succeed, or until it's clear something is stopping you from failing - at which point the caster will tell you to remove whatever that effect is doing this, or have it assumed you are disloyal.

What are all the countermeasures to this spell which:

1. Don't allow the spell to affect you


2. Don't alert the caster you're unaffected

Note that to fulfil (1), answers should avoid "you can lie by telling the truth in a ZoT". A competent interrogator will simply ask direct yes/no questions and assume disloyalty if they are not answered directly.

Mechanically, Zone of Truth is nigh infallible for loyalty tests

As you say, any attempt to resist or evade the effects of Zone of Truth alerts the caster. And in the context of a loyalty test, avoiding the effect is synonymous with admitting disloyalty. As far as I'm aware, there is no way to avoid the effect of the spell without alerting the caster that you have done so. So if you want to make it through the loyalty test, you'll have to do so by telling what your character believes to be the truth.

Believe your lies

However, that leaves at least one small and difficult-to-exploit loophole: Zone of Truth only prevents a creature from speaking a deliberate lie. If you can manage to believe your lies, you will be able to speak those lies while under the effect of Zone of Truth. For example, if the loyalty question is something like "Are you loyal to the Empire?", then you would be able to answer with a simple "yes" even if you were plotting to assassinate the emperor, as long as you truly and honestly believed that doing so would help the Empire. Obviously, this is a very situational workaround, but I want to emphasize that this is materially different from being evasive or "lying with the truth". This is a case where you give exactly the truthful answer that is expected of you, but for a completely different reason (while praying that you are never asked to elaborate on that reason).

In practice, when you as a player attempt to use this, you should probably expect some push-back from your DM, asking if your character really believes this. This "loophole" has a heavy character focus rather than a mechanical focus, so if you are planning to do this, be prepared with a solidly fleshed-out backstory to justify your honesty. And of course, be willing to accept that a slightly different question might reveal that your interpretation of loyalty differs from that of your interrogators. In the above example, you would not fare well if you were instead asked "Are you loyal to the emperor?"

Unfortunately, I don't have any experience to share of using this "loophole" in an actual game. I'm just pointing out that it's the only loophole I can think of.

Modify Memory

Depending on the circumstances and the precise information you need to keep secret, you may able to effectively defeat a Zone of Truth by having your memory modified in advance by an ally using the Modify Memory spell. Affected creatures in a Zone of Truth cannot deliberately lie, but that doesn't mean that whatever they believe to be true actually is. After interrogation, someone surreptitiously casts Remove Curse or Greater Restoration on you and your original memory returns, at which point you presumably mutter "keikaku¹ doori".

Given the restrictions of Modify Memory this works best for concealing knowledge of specific events; the longer ago the events are or the more material you have to cover, the technique swiftly becomes impractical.

This doesn't strictly meet your constraints since it requires you to actually be affected by Zone of Truth and does not conceal that from the caster, but I am not sure there actually is a reliable way to render oneself immune to the spell without tipping off or having to compromise the caster. Either they know you're passing your saves, or by lack of information they know you're not even making saves and therefore aren't affected.

_{1. keikaku means plan}