Identify mysqldump activity in MySQL log file?
SUGGESTION #1
The quickest way to locate any dump activity is the enable the slow query log
You should find queries that resemble something like the following
SELECT /* SQL_NO_CACHE */ from ...

Any large table running a long dump has to show up in the slow log.
If all the tables are small, then lower long_query_time with
SET GLOBAL long_query_time = 0.2;

Let it run for a few hours. Then change it back to 10.
SET GLOBAL long_query_time = 10;

If you don't change it back, the slow log will grow big very quickly.
SUGGESTION #2
Another way would be to check the process list. Please see my 5-year-old post (How to detect a running mysqldump?)
In your case, just run the following query
SELECT * FROM information_schema.processlist
WHERE info like 'SELECT%'
AND   info like '%SQL_NO_CACHE%';

This query will tell you the Process ID, MySQL username, and host doing the mysqldump.
GIVE IT A TRY !!!