I think you are saying that you were able to see steam rising from the washbasin when you looked in the mirror, but not when you looked at it directly in front of you.
I agree with DilithiumMatrix : this is most likely due to colour contrast. The steam is white and the background in front of you (the washbasin and wall) is probably also white. There is very little colour contrast, so your eyes have difficulty seeing the steam. When you look in the mirror the background is your clothing, which is probably not white, maybe even dark colours. There is greater colour contrast making it easier to see the white steam.
To see the effect more dramatically, place a white board behind the steam, and hold a black board in front of the steam so that you can see it in the mirror. If you switch the boards you will be able to see the steam directly but not in the mirror.
Alternatively you could place white and black boards side by side behind the steam and also in front of the steam. You will see steam against the black board but not against the white board, both directly and in the mirror. This shows that it is not the mirror which is making the steam visible.