“It was an ‘anonymous’ survey sent individually to us via company mail...but with serial numbers.”
Company I work for did that....many complained. They changed it...replaced the serial number with your birthdate!!! It was a little better at least because there are about 19,000 employees so there are likely many duplicates. But if you mentioned something specific about your department...you may be pegged.
LOL, I think it went right over the heads of most people where I work. As it is, I trust NOTHING when it comes to surveys!