[aj7]

In subfields of physics, and I suspect math, the submitter is never anonymous. These people talk at conferences, have a list of previous works, etc., and fields are highly specialized. So the reviewer knows with 50-95% certainty who he is reviewing.

[gus_massa]

I agree, also many papers near the begining say

> We are exending our previous work in [7]

or cite a few relevant papers

> This topic has been studied in [3-8]

Where 3 was published by group X, 5 by group Y, 7 by group Z and 4, 6 and 8 by group W. Anyone can guess the author of the paper is in group W.

Just looking at the citations, it's easy to guess the group of the author.

[hexane360]

In many subfields, the submitter isn't even attempted to be hidden from the reviewers. Usually, even the reviewers can be guessed with high accuracy by the submitters