McGurk Gets Good and Mad by E.W. Hildick, 1982.

The members of the McGurk Organization have been planning a kind of open house in order to show the other kids from the neighborhood the kind of mystery-solving work they do. However, someone seems to be doing their best to ruin it.

It starts with the mice that the members of the Organization find in their headquarters, which have already gnawed some of the exhibits for the open house. It quickly becomes obvious that the mice didn’t find their way in by accident when someone posts a sign warning people away from the open house because of the mice. There’s no way anyone could have found out that there were mice, especially not that quickly, unless they had planted them there deliberately. The mysterious sign is signed “Agent 93,” but who is that?

While Brains works on a way to get rid of the mice, the others begin thinking about who could have it in for the McGurk Organization. They consider past cases they’ve solved and villains that they’ve unmasked as well as some of the kids they know generally from school who don’t like them or were offended when they weren’t allowed to join the Organization.

By the time of the open house, they still don’t know who the saboteur is, and when it’s time to unveil the real police handcuffs that Patrolman Cassidy gave them as an award (the highlight of the open house for the McGurk Organization), they are suddenly and inexplicably replaced by a pair of fake mice and another note from “Agent 93.”

Whatever it takes, McGurk is determined to find the guilty person and get those handcuffs back!

The book is currently available online through Internet Archive.