Kautilya's Arthashastra 13.3.48

13.3.48

sārthagaṇavāsibhir ātivāhikaiḥ kanyāvāhikair aśvapaṇyavyavahāribhir upakaraṇahārakair dhānyakretṛvikretṛbhir vā pravrajitaliṅgibhir dūtaiś ca daṇaḍatinayanam, saṃdhikarmaviśvāsanārtham //

Or taking advantage of peace and friendship with the enemy, army and weapons may be collected inside the enemy's fort by spies disguised as merchants, caravans, processions leading a bride, merchants selling horses, peddlers trading in miscellaneous articles, purchasers or sellers of grains, and as ascetics. These and others are the spies aiming on the life of a king.