unexplained
adj.describing something that has no clear reason or cause. You use this when nobody can find a logical way to explain what happened.
adj.lacking a known cause, reason, or explanation. Often used to describe mysterious phenomena or sudden changes in data.
“The police are investigating his unexplained disappearance.”
“The scientists were baffled by the unexplained drop in temperature during the experiment.”
“While most of the symptoms were typical for the virus, the patient exhibited several unexplained neurological signs that suggested a more complex underlying condition.”
Typically used attributively before a noun; when used predicatively, it often follows 'remain' or 'be'.
Learners sometimes try to add a preposition like 'of' or 'about' after the adjective, but it is typically used alone.
“Doctors are being asked to watch out for patients with unexplained fever who work in and around the airport.”
“By far the most common is heavy or unexplained bleeding.”
From un- + explained.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.