Selecting the filter "Not in effect" does not show old, expired. Shows DT's that are scheduled, but not presently "running".
So I looked in Audit logs - old DT's should be there - nope! Interestingly DT's scheduled via API (and now expired) do show (SECURITY). But DT's scheduled via the UI do not.
Even more curious, my attempt to delete a DT via the UI was logged:
User Interface MODIFY gbeyer3 10.2.242.28 User [gbeyer3] submitted a Core UI Proxy command: DEL_HOST_DOWNTIME on Host: -, Service: Greg Beyer
Someone incorrectly submitting a downtime can have negative effects on an enterprise. All DT's should be reviewable in the Downtime tool, and should be captured in the audit log.