Let's talk transmissions codes

After the Transmission Tower quest line, there seem to be six more transmission numbers, that can be found randomly.

The wiki says "Private frequency codes can rarely be found printed on random walls on surface maps. Each code is assigned to a separate map."

So far I haven't see a code. Anyone having better luck than me?

I go into a map thinking I better keep looking out for a code on a wall. 2 minutes in and my mind is at battle and loot, forgetting code search everything.