What's the caster level for the effect that you effectively cast using a spell completion or spell trigger item?
My initial thoughts is that it's based on the items caster level, ie if you use a cleric scroll of cure moderate wounds and its minimum caster level is 3, then the spell when you use this feature is cast as a 3rd level caster.
Your initial thought is correct.
The ability only changes what happens to the item when used. Namely that the item or charge is not expended and you lose your spell slot (or nothing happens).
"If he succeeds, the item’s spell occurs and the item or charge is not expended."
To be clear though, the Caster Level is the caster level of the item which can be higher than the minimum level for the spell.
The exception is staves. They use the better of its CL and yours, and the better of its casting mod and yours. They also use your feats.
This means that staves are far more potent in the hands of a powerful spellcaster. Because they use the wielder's ability score to set the save DC for the spell, spells from a staff are often harder to resist than those from other magic items, which use the minimum ability score required to cast the spell. Not only are aspects of the spell dependent on caster level (range, duration, and so on) potentially higher, but spells from a staff are also harder to dispel and have a better chance of overcoming a target's spell resistance.
Staves hold a maximum of 10 charges. Each spell cast from a staff consumes one or more charges. When a staff runs out of charges, it cannot be used until it is recharged. Each morning, when a spellcaster prepares spells or regains spell slots, he can also imbue one staff with a portion of his power so long as one or more of the spells cast by the staff is on his spell list and he is capable of casting at least one of the spells. Imbuing a staff with this power restores one charge to the staff, but the caster must forgo one prepared spell or spell slot of a level equal to the highest-level spell cast by the staff. For example, a 9th-level wizard with a staff of fire could imbue the staff with one charge per day by using up one of his 4th-level spells. A staff cannot gain more than one charge per day and a caster cannot imbue more than one staff per day.
Furthermore, a staff can hold a spell of any level, unlike a wand, which is limited to spells of 4th level or lower. The minimum caster level of a staff is 8th.