Sinks to the grave with unperceived decay,
while resignation gently slopes the way. [ Goldsmith ]

But on he moves to meet his latter end,
Angels around befriending virtue's friend;
Sinks to the grave with unperceived decay,
While resignation gently slopes the way;
And all his prospects bright'ning to the last,
His heaven commences, ere the world be past! [ Goldsmith ]

An observant man, in all his intercourse with society and the world, carries a pencil constantly in his hand, and, unperceived, marks on every person and thing the figure expressive of its value, and therefore instantly on meeting that person or thing again, knows what kind and degree of attention to give it. This is to make something of experience. [ John Foster ]

Custom is a violent and treacherous school mistress. She, by little and little, slyly and unperceived, slips in the foot of her authority; but having by this gentle and humble beginning, with the benefit of time, fixed and established it, she then unmasks a furious and tyrannic countenance, against which we have no more the courage or the power so much as to lift up our eyes. [ Montaigne ]