D&D 5E Noticing a minor illusion as threat

[guachi]

One is a real thing, the other is a statue.

Since statues don't move it'd be fairly clear it was a statue and not a real person. It also couldn't make sound, smell, or light. Also, the image only lasts for one minute though this brings up the question of how a DM would rule if you cast the exact same illusion all over again. Is there any break between one spell being cast and the next? Can you notice it? On the positive side for the caster is that minor illusion doesn't have verbal components so if you are unseen you can cast the spell and probably not be noticed as you don't have to make sound.

 

[Oofta]

I rule that you can make an immobile figure that looks like a person. If not a shorter race, they have to be sitting, etc. In addition it will not pass much more than a casual inspection.

I give people a free investigation check since the illusory statue doesn't respond to people approaching so it won't pass more than casual inspection.

The very first person to interact with the illusion will of course reveal it as an illusion.

If the PCs immediately attack (why?) then there is no surprise round, although I would give the NPC advantage on a stealth check to hide because they set up a distraction.