It’s possible that he either fell, and the staff found him on the floor, and placed him in his chair, without reporting it, or staff assisting him, dropped him, then didn’t report it and put him in his chair. Unscrupulous staff are not likely to report a mistake or accident. It would be “suspicious” because it’s suspected that either of the two scenarios could have taken place, especially since he was found sitting in his chair. I’ve worked as a nurse, in nursing homes. Some of the staff, nurses AND aides, had appallingly bad characters. (Naturally, there are many good ones, as well.)
What you describe sound logical.
You seem like a very observant Nurse, trained and aware of proper standards. You’re right, there is a certain percentage of Incident Reports that will never be reported in full.
This happens in mental health care as well.