Ever since reading through the Complete Mage, two invocations have stuck with me.
All-Seeing Eyes, granting a +6 to Search and Spot checks for 24 hrs, and 
Otherworldly Whispers, likewise +6 to Knowledge (arcana),(Planes),and(religion).
So let's take a human, add a level of paragon so he can add search to his class skills,
add 5 levels of warlock (I think 6th level seems to be where character concepts gel):
Warlock Detective
STR    8  -1
DEX  13  +1
CON 12  +1
INT   16  +3
WIS   10 
CHA  14  +2
HP 5d6+14 (32hp)
BAB +3
Ranged +4 3d6 Eldritch Blast
FRW 2/2/6
Invocations :All-seeing eyes, Otherworldly whispers,
see the unseen
Decieve Item, DR 1/cold iron, Detect Magic (Sp) CL 5
Bluff +8 
Concentration +4
Disguise +8
Gather Information +5
Intimidate +8
Kn (Arcana) +14
Kn (Planes) +10
Kn (Religion) +10
Search +18
Sense Motive +6
Spot +10
Investigate (Eberron), Investigator, Urban Tracking (UA), Skill Focus (UMD).
Hmm. what does he do, now?
 
