Here is my 100% Achievement guide and full walkthrough!
When we enter the police station for the first time in this chapter, we need to go upstairs to Eddies office and drop off our file.
Once done, head back down, take a right into the canteen where officer crapcut.. haircut.. Greggs!! Is sitting.
Interact with the cobbler on the table and thus begins a long waiting sequence.
You can either talk to Greggs or keep waiting, but keep an eye out on Denise who's just in view.
Hover over the door until you get the prompt to "eavesdrop".
You should here them talking about who's been screwing with the mail.
Once Denise enters the canteen to see you, she says "Guess what? We've found the mailbox bandit"!
At this point, press the
and then the
button to say "It was an animal"
Achievement should unlock.
Go to 28:50 to see the start of where to get this.
Hope this helps!