Currently when the Transaction Driver connection (ie. https://github.com/jbosstm/narayana/blob/5.9.10.Final/ArjunaJTA/jdbc/classes/com/arjuna/ats/internal/jdbc/ConnectionImple.java#L277) is to be closed it tries to delist itself from the transaction (if there is some active transaction in the current thread context).

When the connection was not enlisted into the transaction then WARNing is printed - ie. WARN com.arjuna.ats.jta - ARJUNA016087: TransactionImple.delistResource - unknown resource.
It's only warning and the processing continues without functional issue but the connection could be checking this to not be printing warnings to the log without a good reason.