The chances of finding a unique treasure increase when the meter changes from yellow to red. This goes on until the treasure is guaranteed. However, the meter resets if you leave the area or escape that battle. You can avoid this by ensuring you have energy potions in your inventory.
The treasure finder can be upgraded to silver with a silver ore found in Dimore Cave. After that, it can be upgraded with gold and mithril. This will make the meter charge faster.