mysterious events or situations are difficult to explain or understand:
The police are investigating the mysterious deaths of children at the hospital.
Benson later disappeared in mysterious circumstances.
There's something mysterious going on.
a mysterious person is someone who you know very little about and who seems strange or interesting [= enigmatic]:
I decided to find out more about my mysterious new neighbour.
a mysterious stranger
someone who behaves in a mysterious way says very little about what they are doing, in a way that makes you want to know more [= secretive]
Helen's being very mysterious about her plans.
She hid her thoughts behind a mysterious smile.
Jackson had mysteriously disappeared.