Most noticable with the soviet tesla coil mod, where the sprite is about 3x5, but the foundation size is only 3x3 - the Turret Shield Generator is half off of the turret's base.

Turret shield is always placed in centre of turret's position

Factorio isn't playing nice with PrintScreen, and /screenshot doesn't include outlines from power grids, so I had to do it manually:
https://drive.google.com/file/d/1AU3gS3txV3tBm-eh9Xsk7IpSA3KuNprY/view?usp=sharing
The lighter square is the building's foundation, the darker square is (slightly below) where the turret shield is placed.