There dosen't seem to be any "secret message" as sent by Dhṛtarāṣṭra for Arjuna.
This is what we get from the Book 5: Udyoga Parva, Section XXII.
"Dhritarashtra said, 'They say, O Sanjaya, that the Pandavas have arrived at Upaplavya. Go thou and enquire after them.
Thou must greet Ajatasatru (i.e. Yudhiṣṭhira) in the following words- '...