Most gas stations are franchises. Each franchise has a slightly different contract, including what needs to be reported on a mystery shop. Some items are legal or regulatory requirements, others branding and imaging. An MSC is not going to create different report forms for each franchise. It is much more cost effective and accurate to clean those during editing than confuse shoppers with 100 versions.
Things like porn being sold/visible may not be an issue in certain locations, so it gets changed to a “No”. Same thing with pictures being deleted when they aren’t necessary for a certain location. It’s part of the cleaning process to prep the data for inclusion in the data file. Not something I would personally recommend during the editing phase, but hardly nefarious.
If you ever saw how much your report changed from submission to inclusion, you would be shocked. From the data management perspective, it is far better to simply have thousands of shoppers filling out the same report then making adjustments on the back end. Most schedulers, editors and even project managers are not going to be able to explain it because they don’t really understand what’s involved in data processing.
That’s not to say mistakes don’t happen and someone might accidentally delete the wrong picture (which is one reason not to do it during editing). You should certainly ask if you have concerns about your reports being altered. However, don’t expect an editor to be able to explain something they don’t really understand. This would be a great question to ask in person at one of the shopping conferences.