The suspect, identified as P K Mitra, was arrested after suspicion aroused as he took pictures of the premises.
He is being questioned by a joint team of local police, military intelligence and central security agencies to find out whether he was a spy or not.
"He was suspected to be an impostor and when questioned by security, he was found to be fake. He was later handed over to the police", an army source said.
It is responsible for defending the sensitive northeast and eastern parts of the country.