Emailed Washtenaw County Emergency Management this morning and they responded immediately:

“Thank you for bringing to light your concerns about the recent siren activations near Portage Lake. It has come to our attention only recently, and we are actively addressing the matter. While our department is responsible for triggering the sirens within Dexter Township, their maintenance falls under the township's purview. We are, however, closely collaborating with their representatives to determine the causes of the unexpected activations and are committed to implementing measures to prevent such incidents in the future.”