Can you lose Trust Level 3 without Blizzard notifying you?

I think that’s unlikely.

In the past 24 hours I have probably viewed maybe 20 more topics. But I added about 18k posts to my read count, with about 13k coming in the past 2 hours. And shortly after adding 13k posts to my read count from that test I did, Trust level 3 almost immediately came back.

Signs point to it being the read count.