Definition of "mysteriously"

'Are there any hares about here?' I asked. 'Well, there are some left to be sure,' answered the peasant; 'on the Sukkestad-moor there is an old jack-hare, which many a hound and sportsman have been after, but he is not so easily killed, it appears.' And the worthy peasant shook his head somewhat mysteriously.

1886, Peter Christen Asbjørnsen, translated by H.L. Brækstad, Folk and Fairy Tales, page 76

I put my finger to the side of my nose and nodded mysteriously.

1899 February, Joseph Conrad, “The Heart of Darkness”, in Blackwood’s Edinburgh Magazine, volume CLXV, number M, New York, N.Y.: The Leonard Scott Publishing Company, […], part I, page 219

At that moment a long, scraggy individual in a checked suit poked his head into the bar, looked around portentously, whistled mysteriously to my informant, and jerked his thumb and head in the direction of the door.

1946, George Johnston, Skyscrapers in the Mist, page 121