Sounds similar to watermarking... If you search the forum, there are many solutions to bulk watermarking images.
It is hard to comment on an unknown workflow and or script. All I can do is say how I would do it... It would have either been a script that uses the open event, or more commonly, it would have been a script that was setup using File > Scripts > Scripts Events Manager...
Some points to consider:
* I presume that you have a "master" source file that contains the various layers?
* Is the placed content at an absolute fixed size and position, regardless of the image that it is neing placed on, or do you need the placed content to change size so that it always takes up a certain relative size and position on each image?
* Do you need access to the individual layers in the source image once it has been added to each image? Or is adding the content as a single merged layer enough?
* Are the destination files flattened/single layer or do they have multiple layers (or a mixture of both)? If layered, where in the layer stack should the new placed content be ordered, at the very top or somewhere else?