- If the time interval is near constant, just start a sqlprofiler trace to capture all the needed and analyse it afterward.

- how are the backups being created at your systems ? SQLAgent scheduled jobs ?

check all sqlagent jobs to they don't contain a "backup log with truncate only" statement (we've seen that at some of our systems and had to contact the corresponding dev teams to clarify and remove them )

select J.name, suser_sname(J.owner_sid) as Owner, J.enabled, JS.*

from msdb..sysjobs J

inner join msdb..sysjobsteps JS

on JS.job_id = J.job_id

where JS.command like '%backup%truncate%'

order by Owner, name