My sil and her dh used to work in the same department with a younger female colleague.
They were all friendly and send birthday cards,etc.
Then they all moved jobs. Sil has just discovered her dh had continued to send birthday texts etc to the woman without her knowledge.
She found them by chance and read them. They sound innocent to me. Even when the woman says "hope you are both doing well" he hasn't mentioned it to his wife.
I've advised that he's a friendly type of person and there is nothing to worry about. Sil is concerned about the secrecy. I hope I have given her the correct advice.