It left the solar system faster than gravity says it should.

'Oumuamua was the first interstellar object humans have ever observed inside our solar system. It passed through in late 2017 at speeds that placed it firmly on a hyperbolic exit trajectory — and then it accelerated on the way out, leaving the Sun's neighborhood faster than gravity alone predicts. The leading explanation is outgassing from a comet-like body, but no coma, no tail, and no spectral signature of outgassing were observed. By the time anybody could point a serious instrument at it, it was already gone. No second interstellar object since has had the same anomalous acceleration.