Following along with my Homam Corazza Test Results thread, I'll go ahead and make public the results of my Treasure Hunter testing.
Here was the setup.
Mob Killed: Undisclosed, but all 3,017 mobs killed were the SAME mob in the SAME zone.
Number of times mob was killed: 3,017
Number of kills without Thief's Knife: 1,687
Number of kills with Thief's Knife: 1,330
With Thief's Knife
Drop 1:
---Number of Drops: 279
---Drop Rate: 20.98%
Drop 2:
---Number of Drops: 133
---Drop Rate: 10.00%
Without Thief's Knife
Drop 1:
---Number of Drops: 335
---Drop Rate: 19.86%
Drop 2:
---Number of Drops: 152
---Drop Rate: 9.01%
Along with this, I recorded the exact day and moon phase during every drop, but since the number of divisions of day and moon phase is so high, I don't have enough kills yet to determine a correlation between day / drop rate, and moon / drop rate. Although I can pretty certainly say that Watersday ABSOLUTELY SUCKS for drops. Unless I just had a ton of really bad streaks.
Feel free to make your own conclusions about this, but comparing with other items in the game, I am pretty confident that
Treasure Hunter +x = Drop Rate +x%
is a universal law.
Here are the possible factors that could cause my conclusion to be inaccurate:
1) Weather effects that would sporadically show up in the zone.
2) Kills being too sparsely distributed across the various days and moon phases.
3) Gear being worn, and/or elemental resistances on those gear.
4) Food being eaten (sometimes ate Meat kabob, which increases STR).