Many years ago, I worked in a blue collar shop that had a suggestion box. One morning there was a meeting of all the management and there seemed to be a lot of concern over something. I noticed that the suggestion box disappeared around this time, but did not associate the two events. When I was leaving that facility for another within the same company, they told me that they hadn't wanted to mention it, but a very suggestive suggestion had been placed in the suggestion box and they felt it should be removed at that time. Not quite understanding why they were telling me this, I asked what they meant. The said the suggestion was about me. For a part of the time I worked there, I worked the 3:30 to midnight shift and was alone on that side of the building for most of the time. I wasn't very happy, but young enough to blow it off then. In retrospect, they really probably should have done something like let me know then so I would be more observant. There were approximately 350 workers at the facility - odds are that at least one was a nut.
I have disliked suggestion boxes since that time. I think people lose control with written word and anonymity, so I think the third party telephone method would be best.