Detective is a Gold trophy in Portal of Evil: Stolen Runes. It can be received for finding all the letters from grandpa.
There are 15 letters from grandpa spread out in the main story. They come in golden metal canisters, can be found in following locations in order :

1) Garden Fountain – In the secret compartment with the pass card
2) Gates (past ruby door) – In compartment with UV lamp
3) Portals Room – On stairs on the left. It's almost completely covered by the canister icon above your journal.
4) First Portal - Mountain – Against the rock, between the campfire and broken stairs
5) First Portal – Caves – On the ground next to the pots
6) Second Portal – Bridge – Against the tree, next to the left bridge post
7) Second Portal – Inside Temple – On the stairs
8) Second Portal – Cave – Directly to the right of the canister icon above your journal
9) Basement – In with the mask half and the skull
10) Third Portal – Bridges – In bookshelves under left chandelier
11) Third Portal – Repository - Behind the pillar that has the jade dragon on it, above the journal/canister icons
12) Third Portal – Archives – On the stairs leading up to the desk
13) Fourth Portal – Hallway – Right above hint button/map candles icons
14) Fourth Portal – Dungeon – In one of the caskets on the right
15) Fourth Portal – Study – Behind the painting