That might have been OTA update.
I had 4 locked S3's.
1st one: SHV-E210L: locked to LG U+ (South Korea), I flashed stock 4.3 and phone remained unlocked.
2nd one: GT-I9300: locked to Vodafone UK, I flashed stock 4.1.2, phone remained unlocked.
3rd one: GT-I9300: locked to ORANGE UK, I flashed stock 4.1.2, phone remained unlocked.
4th one: SGH-I747M: locked to Rogers Canada, flashed stock 4.3, phone remained unlocked.
Updating can not re-lock an android phone that has already been unlocked.
Re-locking only happens on iPhones.
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