This is the encoding of the Subject line, or part of the subject line.
This should not normally be visible. It could have been incorrectly
applied by the sender's mail client, the sender's mail server, or even
your incoming mail server.

The tag you see should be at the beginning of the Unicode encoded text,
and it should end with something like =?=.

Can you post the entire Subject line from the actual message headers;
View -> Options (of course omitting any sensitive information)?