Purposeful Undead are active and have blazing eyes, whereas zombies are just dead.
The description of a Revenant, a purpose-driven undead, states (MM 259)
The revenant reclaims its mortal body and superficially resembles a zombie. However, instead of lifeless eyes, a revenant's eyes burn with resolve and flare in the presence of its adversary.
Likewise, the purposeful, but evil Wight also has glowing eyes (MM 360)
When a wight attacks, this life essence glows like white- hot embers to its dark eyes, and the wight's cold touch can drain the spark through flesh, clothing, and armor.
On the other hand, a zombie is less "alive" (MM 315):
A zombie retains no vestiges of its former self, its mind devoid of thought and imagination. A zombie left without orders simply stands in place and rots unless something comes along that it can kill.
Likewise, while zombies merely stand and rot until commanded (INT 3), purposeful undead like the Revenant are smarter (INT 13).
Therefore, especially from a distance, it might be difficult for your players to even tell that "faithful guardian" undead are even undead, aside from specific undead detection methods. The guardians could continue to behave like ordinary guards, albeit with strange glowing eyes, that make patrols, interact with each other, or maybe even clean their surroundings.
There's probably no way to explicitly tell your players that these undead are not "bad", but making them more lifelike will give your players pause.
Tell your players what your characters know
Out of character, I find it useful to simply tell players what their characters know. For example, in my game, I simply tell my players stuff like, "You only know of 4 elemental planes, so you think it's unusual that this guy is mentioning a fifth". In your case, you could tell your players "These undead do not seem to be behaving like any undead you've seen before". Such knowledge would be obvious to the characters, having lived in that fictional world, but it's not obvious to the players, who might not know the specifics of your fantasy world.
I find that these kinds of statements actually benefit immersion, because then your players can control characters that have existed in the world. Otherwise, you end up with players using their IRL biases, like "all undead are evil," in a world where their characters, who may have interacted with undead, would know better.