Hohige
Explorer
there isn't any direct rule about it on combat.It's not undetectable as I would also rule that a simple Wisdom (Insight) check is enough to determine if someone is doing something.
When you interact with an illusion, requires your action (Investigaion check) to perceive the illusion.
To find something hidden, requires an Search action (Perception check). On combat, all requires your action, unless descripted.
But here, It doesn't have any evidence about mind controlling.
As a DM. You certainly must use your action to roll a Insight check. But, without any evidence, I wouldn't call a Insight check.