My wife and I had been married for about 25 years when I noticed that she had started masturbating frequently in bed after she thought I was asleep. We had always had a very low key sex life and I didn't think she enjoyed sex, she certainly didn't give me any pleasure over and above what felt like a "duty" fuck once a month. Like your wife mine denied she was doing it. She was not nearly so noisy about it, she was much more discrete, she had never been a "moaner". On those occasions when I reached over to her (and this was never appreciated or encouraged however gently I tried) and it was obvious what she was doing, she would first deny it then say she couldn't sleep or she was doing it to release stress.
Finally after about a year I put two and two together and realized most of her nightly sessions coincided with visits with her Spanish teacher (started with one lesson a week but increased over the year so that she was seeing him 3 times in the week and often on the week end when she told me was visiting girlfriends).
I always found her masturbating incredibly sexy and hoped we might use it to improve our sex life. It turned out to be another symptom of the death of our marriage. Even when she admitted she had started fucking the teacher I couldn't help being pleased for her as she said she was enjoying sex for the first time in her life. Our problems could not be sorted out in therapy and within two years we separated.