I think a better questions might be "why are attachments getting attached to the wrong customers?". And if you use the standard TAM temporary storage area, you are sharing that folder with everyone in the office. So if you scan a document, it is temporarily stored there for everyone to see, probably with a very non-descriptive file name. So anyone who is attaching thinks it is their scan, and attaches it in the normal workflow. I would look at who attached these errant scans, and if it is someone who wouldn't normally work on that customer, chances are good that this might be the case.
Each user in our office has their own folder and only that user can see that file and attach it. It saves tons of grief like this. If you find that to be the case, either I or someone else here can show you how to fix this very quickly and easily.