that's an application error,and not SQL server.
a lot of applications will have a default connection timeout of 30 seconds, unless someone explicitly adds some longer timeouts to the connection string.
if the app then runs a query that is long running, the application throws it's hands up after thirty seconds and returns an error, even though SQL server is still working on returning the results/finishing the desired query.
you could check the DMV's for long running queries, but it would be better to throw this in the lap of the applications team and tell them they are timing out ; have the original user report what exactly they were running that took a long time.
this might help find some of your slower queries:
from sys.dm_exec_query_stats st
cross apply sys.dm_exec_sql_text(st.[sql_handle]) fn
where st.max_elapsed_time >= (1000 * 15) --15 seconds
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