How much effort do you want to expand in tracking down a villain? The plan is for this to a rather big part of the game in fact the purpose of the game is track down a type of cyber criminal as such it can't be easy otherwise the player won't feel that they have accomplished anything and it can't be too difficult otherwise player might become frustrated by their lack of progress. It also has to be interesting enough for the player to repeat over and over again. I'm thinking that tracking down a criminal should be almost like solving a jigsaw puzzle where you have to find the pieces, and I'm thinking of displaying the clues amassed in the hunt in a similar matter. The question is though should the player have to search for the clues or always find them upon completing a mission? Case in point, you've learned that a vicious criminal is using a group of zombies to raid the warehouse of high tech company, so you try and stop them. Now one of the zombies has a cell phone which still contains the number from which the zombie’s activation single was sent from. Finding this number is a vital clue in your search for this criminal, since it is the start of a trail you can follow. Now how much work should the player have to do to get that phone? Of the following levels of work which would you consider optimal? 1) After completing the mission you automatically discover the phone number. 2) After defeating the zombie carrying it you find the phone number. 3) After defeating the zombie if you search it you can find the cell phone. 4) If you have a green tooth remote connector you can attempt to hack into the cell phone if you can get close enough, assuming that you also have a green tooth snooper to detect the cell phone in the first place. 5) Buy out - Instead of tracking down clues yourself you can buy them off other npcs.

Let the player know that there's a good chance one of the zombies has the clue on their person, then make them search the corpse.

For some good ideas here, look at the newest Splinter Cell game, where you can find things on computers, or interrogate bad guysor whatever.

Another good one is Second Sight, particularly the level called "Madness". There's a section where you break into an insane asylum looking for an old friend. You can pretend to be a psychologist and talk to the other doctors to learn what you need to know, or you can gain access to the computer system and find info that way, or you can access the CCTV network and look for your objective, or you can choke a guy until he tells you what's up. Brilliant.

Basically, if you arrange for the clue to be present, and somehow require the player to get close to the clue (In Second Sight, the control to unlock a door is on a computer that also contains some handy clue data, so you have to look at the screen at east once, and it's one click away), then you can call that reasonable.

Don't put a file folder at the bottom of a mine shaft a mile away from any reasonable location where it should be, and expect players to find it without GameFaqs and some swearing.

Don't require a keycode and have only one dude in the whole level know it (Splinter Cell does well by having one clue for each four or five enemies, and any one of them will give it to you).

Don't let players burn bridges by accidentally shooting a computer during a firefight (*cough* Goldeneye *cough*).

Play some Carmen Sandiego games if you get the chance. They're fun and take some noodling, which is a tough balance to produce. Even if you don't use clues like "Yeah, she said she was going to a city that once has a violent political coup called a 'defenestration'," you can see some winning formulas in those old games.