Good question! 'Whose' always implies possession. For example: Whose book is this? Whose pencil did you take? So, regarding your specific context, you used each variant correctly. If you are speaking about a person in general, you will use "who", but if you are speaking about something that belongs to somebody (i.e the job of person who catches thieves) then you will use "whose", because the job belongs to the man. I hope this help!

whose <--- belongs to