The next chapter specifically states that only then did Mr and Mrs Gardiner realise there may be an attachment.

Her uncle and aunt were all amazement; and the embarrassment of her manner as she spoke, joined to the circumstance itself, and many of the circumstances of the preceding day, opened to them a new idea on the business. Nothing had ever suggested it before, but they now felt that there was no other way of accounting for such attentions from such a quarter than by supposing a partiality for their niece. (chpt 44).