What is the English word for the role of a person who betrays his master's attitudes? [closed]

A Polish diplomat gave Catherine the Great a little dog, then, by observing the little dog, he found out who was Catherine's secret lover.

In a hierarchical organization, people who are two layers above you always look nice, but the ones appointed immediately above you always betray their masters' attitudes.

I wonder if there is an English word for this role played by little dogs.

Thanks,

While not a noun to describe the dog itself, the idiomatic dead giveaway serves your purpose quite well:

Something that reveals a fact or an intention completely. The car in the driveway was a dead giveaway that someone was at home.

Alternatively, just giveaway can be sufficient:

Something that reveals a fact that was meant to be concealed. (Often with dead.) The way he was walking was a giveaway to the fact that he was the one who was injured.

This particularly emphasises the idea that the fact was intended to remain hidden.