Here’s the passage where Miss Bentley tells Major Wilshaw what she noticed about Miss Beaumont in the bathing hut (my emphasis):
Laughing again, she pulled the face of the major down to her, putting her mouth against one of his small pink ears.
“Good God,” he said. “No.”
“You see how unobservant men really are.”
He drew slightly away from her, looking down at her body, seeing it not only as it lay there in the sunlight of afternoon but also as he had seen it, floating, unexpectedly splendid, in the lake below.
“Much depends,” he said, “on what there is to observe.”
She laughed again. With rising excitement he felt his hands slip to her breasts.
H. E. Bates (1960). ‘A Month by the Lake’. In The Grapes of Paradise, p. 133. Boston: Little, Brown.
This indicates that Miss Bentley had noticed something about Miss Beaumont’s breasts. Here’s what the story has to say about Miss Beaumont’s body:
The figure of Miss Beaumont was virginal, slender and wiry, with small, sharp, up-pointed breasts. [p. 89]
Seeing her, he was unaccountably depressed by an effect of flatness about the dark red costume. The legs were extraordinarily thin, like a boy's, and too hollow at the thighs. He experienced the impression that Miss Beaumont, who looked so arrestingly pretty in cool silk frocks, now looked meagre, a mere slice of a girl, skimpy. [pp. 91–92]
The two-piece band poured music down his throat from a distance of two yards like a blast from a furnace door. The hard thin body of Miss Beaumont flung him from side to side. Her breasts pressed against him with the hardness of two shapely little Easter eggs and now and then she chanted into his ear, in an abrasive voice, the brittle, chirpy words of songs she knew. [p. 105]
So it seems likely that what Miss Bentley saw in the bathing hut was that Miss Beaumont was flat-chested and wore a padded bra.