it's been a long time coming...
I bet you'd never have thought you'd get a reply over 2 years after you posted this...
If it's still at all useful, I did make up an Arcane Investigator PrC a few years ago:
Arcane Investigator
Arcane investigators are the preeminent sleuths, bringing their keen senses, powers of deduction and divinations to bear in solving cases or thwarting the plans of villains. As such, they are often sought for their talents and charged with the unraveling of mysterious occurrences and the finding of lost items or people. Most arcane investigators are wizards, particularly diviners, with sorcerers and bards sometimes taking up the mantle of the arcane investigator. Rogue/wizards are particularly effective arcane investigators, combining their magical and mundane skills to delve into the unknown.
Level BAB Fort Save Ref Special Spells Per Day
1st +0 +0 +0 +2 Trapfinding +1 level
2nd +1 +0 +0 +3 Resist Illusion +1 level
3rd +1 +1 +1 +3 Uncanny Dodge +1 level
4th +2 +1 +1 +4 Trap Sense +1 +1 level
5th +2 +1 +1 +4 Leap of Logic 1/week +1 level
6th +3 +2 +2 +5 Unveil +1 level
7th +3 +2 +2 +5 Trap Sense +2 +1 level
8th +4 +2 +2 +6 Leap of Logic 2/week +1 level
9th +4 +3 +3 +6 Discern Lies +1 level
10th +5 +3 +3 +7 Trap Sense +3, True Seeing +1 level
Game Rule Information
Abilities
Intelligence, Wisdom, and Charisma are important due to the skills that rely upon these stats. Depending on what type of spellcaster the arcane investigator is, one of the three aforementioned attributes would be more highly valued than the others. Dexterity is important because arcane investigators tend to wear light or no armor.
Alignment Any.
Hit Die d4
Requirements
Craft (Trapmaking) 4 ranks
Gather Information 4 ranks
Search 6 ranks
Sense Motive 4 ranks
Feats Investigator, Skill Focus (Craft: trapmaking)
Spells Ability to cast Divination spells of 3rd level or higher
Class Skills
The arcane investigator’s class skills (and the key ability for each skill) are Appraise (Int), Concentration (Con), Craft (Int), Decipher Script (Int), Diplomacy (Cha), Gather Information (Cha), Intimidate (Cha), Knowledge (Int), Listen (Wis), Profession (Wis), Search (Int), Sense Motive (Wis), Spellcraft (Int) and Spot (Wis).
Skill Points at Each Level: 4 + Int modifier.
Class Features
All of the following are class features of the arcane investigator prestige class.
Weapon and Armor Proficiencies Arcane investigators gain no additional proficiency in any weapon or armor.
Spells per Day When an arcane investigator level is gained, the character gains new spells per day as if he had also gained a level in a spellcasting class he belonged to before adding the prestige class. He does not, however, gain any other benefit a character of that class would have gained (feats, hit points), except for an increased effective level of spellcasting. If the character had more than one spellcasting class before becoming an arcane investigator, he must decide to which class he adds the new level when determining spells per day.
Trapfinding (Ex) Arcane Investigators may use the search skill to locate traps as a per the rogue ability.
Resist Illusion (Ex) Arcane investigators of 2nd level or higher gain a +2 insight bonus to all saves versus illusions.
Uncanny Dodge (Ex) At 3rd level, the arcane investigator retains his dexterity bonus to AC (if any) regardless of being caught flat-footed or struck by an invisible attacker. If he had uncanny dodge from a previous class, he automatically gains improved uncanny dodge instead.
Trap Sense (Ex) This ability functions exactly as the rogue ability of the same name and stacks with other Trap Sense bonuses.
Leap of Logic (Sp) At 5th level, an arcane investigator may use their powers of deductive and inductive reasoning to glean important information from seemingly insignificant or unrelated clues and events. By succeeding at a gather information check (DC 20), the arcane investigator gains benefits that mirror the effects of a divination spell. This ability may be attempted once per week at 5th level and twice per week at 8th level.
Unveil (Su) At 6th level, the arcane investigator gains the supernatural ability to penetrate illusions, nondetection spells, and disguises at will. The arcane investigator must touch the illusion, the warded creature, or the creature shrouded in illusion (such as a disguise self spell). The arcane investigator then makes a caster level check as if casting dispel magic against the illusion effect. If he succeeds at this check, the illusion is immediately dispelled. Arcane investigators gain a +2 competence bonus to this counterspell roll and a +4 competence bonus on spot checks against the disguise skill.
Discern Lies (Sp) An arcane investigator of 9th level can use discern lies as a spell-like ability once per day.
True Seeing (Sp) At 10th level the arcane investigator may use true seeing as a spell-like ability once per day.
