Secret police are used to stifle dissent; they aren't subject to it. The whole point of a "secret police force" is that they are an incredibly powerful tool not subject to public oversight, public debate, or even public approval.
Right? If the public knew about it and debated it, it's not a "secret".
The "secret" part of "secret police" refers to the hidden identity of the members of the secret police, not that the mere existence of the secret police is a secret.
If the public didn't know it existed at all, it would be both less effective as a means to limit fringe behavior (police states control through fear of not knowing who is working/informing for them), and would endanger the members when they tried to exercise police functions.