I figured it out yesterday. First you need to talk to Sergent Blueey. (policeman in
Burramudge) He'll talk about a spy. Use a Infrarang or X-rang to identify the spy right by
Dennis's house. Once you kill it, talk to Sargent Blueey again. Now you must talk to
Ranger Ken after beating the crocodile mission. He'll say that there are many more spies
through all over the southern rivers. After you talk to him, you now are able to attack
contruction worker spies using the infrarang. Check in all levels. There will be
construction disguises everywhere. That's how you will collect construction workers on
your game progress menu.

I still don't get how you do it because every site I get on it says to do something
totally different. Just saying. Later.