Don't buy eavesdropping detection gadgets. Don't play detective. Don't hire a private detective and let them play debugging expert. This is serious business. Counterespionage work is a full-time specialty within the security field.
Professional help is available.
Contact an independent security consultant who specializes in electronic eavesdropping detection and espionage prevention. Recognizing this person may not always be easy. There are too few of them, and too many pretenders. Contact several corporate security directors. See who they are using.