Ujm- I agree that it seems like that Shomrim may have jumped the gun. But the mother was genuinely nervous as all it takes is six seconds for an abduction to take place. and she asked the experts. They advised her to call the cops. I don’t think she did anything wrong unless she misrepresented the situation. I’m not sure why Shomrim thought calling the police was the right call here… I’m definitely open to accusing them of making a serious mistake, but it’s hard for me to say that with any kind of confidence without knowing their reasoning.
Is it the issue of mesira? Depends on whether the gedarim of mesira extends to a reasonably just society. I’m not convinced it does… But whether it’s the formally issur of mesira or not, a false accusation is a serious thing.