Briar shares your hardware’s Bluetooth MAC address with your contacts even if you haven’t added that contact or connected with that contact using Bluetooth. meaning all your briar contacts even those on groups are storing your MAC address ( a unique device identifier) which makes plausible deniability virtual impossible. I remember reading the devs themselves acknowledge that briar isn’t a anonymous chat app (somewhere in their git repo or the wiki or the blog, can’t remember where exactly), so beware of this aspect.