Banana
Not sure he knows. He did it once because he could, and now he does it if the opportunity arises.
Sounds a bit more like a compulsion, then. A bit of kleptomania, maybe from some underlying psychological whatits, but maybe just a bit of a twitch. Make it your character's Flaw! Get Inspiration when you do it!
People for me are complex with many layers. Just wanting to replicate that into my character really. There is no overwhelming drive or meaning to understand this. It just is.
Someone earlier in this thread mentioned that they didn't think it important to bracket a PC into a specific alignment, a label if you were. I'm starting to agree.
PC's can be more complex than that if you want. Maybe they should be aligned on their consistent morals (especially in a crisis) added to their current approaches. I haven't worded that eloquently, I'm sure someone can put it better than me.
You don't HAVE to put your character in an alignment bucket, really. Alignment's mostly a tool for determining how your character acts, a sort of reference point to remember, hey, your guy DOES care about justice and law and order and DOESN'T just want to inflict suffering. It's OK at that, but it's not exactly a whole nuanced picture. That's what stuff like the Bonds/Flaws/Traits/Ideals are all about.
Sounds to me like you're probably an LN (or even LG, if you're truly ashamed!) copper with the Flaw of "I'm compelled to steal things from the criminals I investigate."
You might think about what would get your character to give up that practice - what would it take to STOP stealing things? What do they get out of it? What need does it fill for you?
Q's like that are part of developing a complex character. "Sometimes I steal stuff" isn't a deeply considered character trait, but "I like to have trophies of my victories, so sometimes I steal stuff" is better. Even more juicy: "My father always criticized me, and I sometimes think I don't deserve the success I have as an investigator. But when I solve a crime, I feel
good about it, because I'm good at my job. I don't want that feeling to end, so I yank a souvenir from my targets. They're awful people who won't need it anymore, and seeing that necklace or bit of coin inspires me to get out and catch the next one. It's what I use to keep the memory of my critical father at bay."
That also helps inform more about what alignment you are. The most juicy description could have an entirely different alignment than the surface-level description (I'd call the most detailed guy LG, fighting against his own internalized criticism for the good of the town).