Something which is kept a secret or kept under covers is called something concealed or disguised.

Covert (kove..t) means the same; something concealed.
The local people feel there are some covert forces behind the strange incidents happening at the bridge.

Overt is the semantic opposite of Covert.
Overt means something not concealed or open to view.
The hostility for the tribals among the villagers is quite overt.