Welcome @Khorshed_Alam, it points that you may have an integration running on the Legacy API Key, provided that report is based on the Legacy API Key usage?
Did you get an email from Keap indicating the number of API Calls that were done in the previous 30 day period? If so, can you report on the number of API Calls being done.
You ask Keap Support to give a detailed API Calls List that contain the IP Addresses. You can then check online to see where they are originating from.
I have written a guide via the link below to help identify things.