Yeah, strictly speaking, caster 12, though the DC is absolute
minimum, which gimps the weapon a lot since not many targeted spells swell with level.
Saving Throws Against Magic Item Powers
Magic items produce spells or spell-like effects. For a saving throw against a spell or spell-like effect from a magic item,
the DC is 10 + the level of the spell or effect + the ability modifier of the minimum ability score needed to cast that level of spell.
Staffs are an exception to the rule. Treat the saving throw as if the wielder cast the spell, including caster level and all modifiers to save DC.
Most item descriptions give saving throw DCs for various effects, particularly when the effect has no exact spell equivalent (making its level otherwise difficult to determine quickly).
Caster Level
The next item in a notational entry gives the caster level of the item, indicating its relative power. The caster level determines the item’s saving throw bonus,
as well as range or other level-dependent aspects of the powers of the item (if variable). It also determines the level that must be contended with should the item come under the effect of a dispel magic spell or similar situation. This information is given in the form "CL x," where "CL" is an abbreviation for caster level and "x" is an ordinal number representing the caster level itself.
For potions, scrolls, and wands, the creator can set the caster level of an item at any number high enough to cast the stored spell and not higher than her own caster level. For other magic items, the caster level is determined by the creator. The minimum caster level is that which is needed to meet the prerequisites given
Spell Storing
A spell storing weapon allows a spellcaster to store a
single targeted spell of up to 3rd level in the weapon. (The spell must have a casting time of 1 standard action.) Any time the weapon strikes a creature and the creature takes damage from it,
the weapon can immediately cast the spell on that creature as a free action if the wielder desires. (This special ability is an exception to the general rule that casting a spell from an item takes at least as long as casting that spell normally.) Once the spell has been cast from the weapon, a spellcaster can cast any other targeted spell of up to 3rd level into it. The weapon magically imparts to the wielder the name of the spell currently stored within it. A randomly rolled spell storing weapon has a 50% chance to have a spell stored in it already.
Strong evocation (plus aura of stored spell); CL 12th; Craft Magic Arms and Armor, creator must be a caster of at least 12th level; Price +1 bonus.