I can think of a couple of things. I sometimes put money in my mother's safe because she is struggling financially, and if I offered it to her she would refuse to take it. I don't think she knows who is doing it.

I also proof-read and drastically altered a friend's essay, without telling them. She is a young mother, and even with me staying with her that night and helping her out she was so exhausted that she fell asleep in the morning, with a noon deadline, with only a first draft completed. I fixed it before sending it for her (it was an incoherent mess), but I just told her I'd sent it to stop her missing the deadline. I don't think she ever knew that the mark she got was because I re-wrote it, but she was so proud of herself that I decided not to tell her. I realise this is probably 'wrong', but she was a high achiever academically and I just wanted to help.