Contacts with blank fields are created automaticly

Hi,

We are on 2015,periodicly unknow process is creating blank contacts in Crm.We suspect it's an issue in Outlook -Crm synchronisation. We don't have any workflows generating contacts, but I noticed for some users it's happened after I tested and enabled their mailbox, which basically force synchronisation for appointments and contacts.Did anybody face a similar issue?

Thanks!