Warnings ARJUNA16037, ARJUNA16038 in log file

We keep seeing the following warning messages in our log files every 2m, 10s:

12:05:29,290 WARN [com.arjuna.ats.jta] (Periodic Recovery) ARJUNA16037: Could not find new XAResource to use for recovering non-serializable XAResource XAResourceRecord < resource:null, txid:< formatId=131076, gtrid_length=29, bqual_length=28, tx_uid=0:ffffac146de0:3231b1f3:4ea67b21:4acc, node_name=1, branch_uid=0:ffffac146de0:3231b1f3:4ea67b21:4ad5, eis_name=unknown eis name >, heuristic: TwoPhaseOutcome.FINISH_OK com.arjuna.ats.internal.jta.resources.arjunacore.XAResourceRecord@79453076 >

On another system these are intermixed with
14:41:27,440 WARN [com.arjuna.ats.jta] (Periodic Recovery) ARJUNA16038: No XAResource to recover < formatId=131076, gtrid_length=29, bqual_length=28, tx_uid=0:ffffac1448a6:-7e9454f8:4e86f7c3:86a7, node_name=1, branch_uid=0:ffffac1448a6:-7e9454f8:4e86f7c3:86ab, eis_name=unknown eis name >

We are using JBoss 7 in standalone mode, with three datasources. One of them is an XA datasource (a H2 database with class org.h2.jdbcx.JdbcDataSource on one system, oracle.jdbc.xa.client.OracleXADataSource on another). JDBC drivers for Oracle and H2 are deployed as jars in standalone/deployments along with our app.