How is her secret activity discovered in The Possibility of Evil?

Miss Strangeworth has been writing her anonymous letters for one year and creating many kinds of bad feelings in the town. On the day covered by the story she writes three more poison-pen letters on sheets of paper of different colors and places them in matching envelopes. One of them will have serious consequences for her.

After thinking for a minute, although she had been phrasing the letter in the back of her mind all the way home, she wrote on a pink sheet: DIDN'T YOU EVER SEE AN IDIOT CHILD BEFORE? SOME PEOPLE JUST SHOULDN'T HAVE CHILDREN SHOULD THEY?

She addressed an envelope to Don Crane after a moment's thought, wondering curiously if he would show the letter to his wife, and using a pink envelope to match the pink paper. 

Evidently Miss Strangeworth has not written to Don or his wife Helen before this. She talked to Helen earlier in the day outside the grocery story, and she is well aware that both husband and wife are seriously concerned about the apparent slow development of their six-month-old baby daughter.

Miss Strangeworth always mails her letters at the post office in the early evening. The small-town post office is always closed by that time, but there is a slot in the front door for people to deposit letters. She pushes two of her letters through the slot, but she inadvertently lets the one in the pink envelope fall to the ground outside. The post office is a hangout for young people in the early evening. One of them sees the pink envelope and tries to attract Miss Strangeworth's attention, but she doesn't hear him as she goes on her way.

The boy who finds the letter happens to be Dave Harris, one of the victims of Miss Strangeworth's letters. She has written to his girlfriend Linda Stewart's parents hinting that Dave and Linda are going far beyond the usual teenage hugging and kissing. Linda's irate father has forbidden Dave to come to their house and is trying to break the young couple up completely. 

Dave decides he will hand-deliver the pink letter to Don Crane. Naturally there is no return address on the envelope, but Dave is sure to tell him Don that the letter was dropped in front of the post-office door by Miss Strangeworth. When Don reads it he will be the first person in town to know that this sweet little old lady is capable of writing such letters.

Don will not know that anybody else has received Miss Strangeworth's anonymous letters, and he may never tell anybody except his wife about her letter to him. He takes his secret revenge by chopping up all her treasured rose bushes and sending her his own anonymous letter which reads:

LOOK OUT AT WHAT USED TO BE YOUR ROSES.

Miss Strangeworth does not even know that this vandalism was a retaliation for anything she had done or that she is in danger of exposure. She just takes it as an example of the widespread evil she knows exists in her town. She may continue writing her poison-pen letters as before. But now that two people, Don and Helen Crane, know her secret, it may not be long before everybody in town will know who has been causing so much trouble.