Understanding Society User Support

I was trying to look at who changed jobs using waves 2, 3, and 4 of Understanding Society. For this I looked at whether the respondent was still in the same job, with the same employer, and at the same workplace. These are three different variables in the questionnaire. Only on one of these three variables, namely whether the respondent is still with the same employer, some respondents are coded -9 (missing). According to the user manual this means the answer is “missing by error or implausible”. Is it known why this occurred? Thanks in advance for you reply.