RECOVERY MODEL changed on a database, no record in Default Trace
Answering my own post as I found additional information:
The trace log had no indication of Object: Altered for that database, which happened to be msdb (might be important here). In the SQL logs, I found indications that someone had applied updates, and after the first instance start-up, there was indication that msdb had an upgrade script applied then setting of SIMPLE recovery, along with creation of several synonyms.