The low-stakes character of international large-scale educational assessments implies that a participating student might at times provide unrelated answers as if s/he was not even reading the items and choosing a response option randomly throughout. Depending on the severity of this invalid response behavior, interpretations of the assessment results are at risk of being invalidated. Not much is known about the prevalence nor impact of such "random responders" in the context of international large-scale educational assessments. Following a mixture item response theory (IRT) approach, an initial investigation of both issues is conducted for the Confidence in and Value of Mathematics/Science (VoM/VoS) scales in the Trends in International Mathematics and Science Study (TIMSS) 2015 student questionnaire. We end with a call to facilitate further mapping of invalid response behavior in this context by the inclusion of instructed response items and survey completion speed indicators in the assessments and a habit of sensitivity checks in all secondary data studies.