DEAR ABBY: I’m writing about “Not Nosy, Just Concerned” (Nov. 13), the neighbor who suspects the woman she sees entering “John” and “Marcia’s” house is having an affair with John while Marcia sleeps. There is another scenario to consider, and it is, perhaps, the biggest reason “Nosy” should keep hers out of it. It is entirely possible that Marcia is fully aware and willingly participating in the after-hours visits of the “tart.”

I am happily married now, but when I was single, I was an after-dark visitor to a well-to-do, (still) happily married couple in an unsuspecting affluent suburban neighborhood. We were all happy with the arrangement, and even joked with each other about what the neighbors would think if they only knew. “Nosy” may get the surprise of her life if she ignores your advice and shares her “knowledge” with Marcia. Things aren’t always as they appear. — HAPPY THIRD WHEEL

DEAR HAPPY: That’s true. And you were by no means the only reader who suggested that scenario. However, many others were quick to offer me some neighborly advice. Read on:

DEAR ABBY: You were off-base in your reply to that lady. She wanted to know how to tell the woman of the house her husband was up to no good. You advised her to mind her own business. You should have told her to send the woman a letter.

If it turns out the visitor is a relative, they can have a good laugh over it and she won’t have messed up her friendship with the couple. If it turns out she’s right, then Marcia will know to get herself tested for an STD, which John may have given her. And if he hasn’t given her one yet, it could stop her from getting one. STDs can kill. Telling the neighbor to butt out was just plain dumb. — FREDA IN FLORIDA

DEAR ABBY: You are out of touch if you don’t think a man will have sex in the same house in which his wife is sleeping. My sister awoke one morning to find her (now ex) husband having sex with their houseguest in the bathroom! And about 15 years ago, a friend of mine caught her husband in the basement with another woman. She and their two children had been upstairs sleeping.

Open your eyes, Abby. Why would a relative come in the middle of the night and park a block away? — DIANE IN MINNESOTA

DEAR ABBY: My mom had the same suspicion years ago. Her neighbor lady’s husband would leave for work and shortly after his departure a man next door would walk over to her house. He’d go through the back gate, into her home, stay awhile and then walk back to his house. After getting to know them, Mom discovered that the woman and her visitor were brother and sister and enjoyed drinking their morning coffee together. — NOSY NO MORE

DEAR ABBY: I wish a nosy neighbor had told me when my fiance was cheating on me. After I found out, several people admitted they had suspected the woman who had been coming around when I was visiting my parents was the “other woman,” but they didn’t want to seem intrusive.

While all of this was going on, I was pregnant with our second child. I learned what was happening when the baby was 3 months old. The woman was someone I considered a friend. I felt betrayed. Abby, “Nosy” should send her neighbor an anonymous note. — SOMEONE WHO’S BEEN THERE

DEAR ABBY: Several years ago my dad came to San Francisco to attend a CPA convention. Because my stepmom, “Ann,” couldn’t be there, Dad invited me to the Saturday night dinner. I was in my early 20s and working as a fashion model, so I decided to dress to kill.

Even though Dad introduced me as his daughter, several of the wives were suspicious. One of them took a picture of us and showed it to Ann. The look on her face was priceless when my stepmom said, “Isn’t ‘Jim’s’ daughter beautiful? I’m so glad she could be there.” The three of us have laughed about it for years. — NORMA IN SAN MATEO, CALIF.

Dear Abby is written by Abigail Van Buren, also known as Jeanne Phillips, and was founded by her mother, Pauline Phillips. Write Dear Abby at www.DearAbby.com or P.O. Box 69440, Los Angeles, CA 90069.