Masturbation

My boyfriend seems to mastubate a lot, he tends to hid it all the time. I know he does it and we've talked about it, but I recently discovered that he does it at work too. Idk if I should be concerned or what. During an argument he said that it isn't the first time he's done it. I honestly don't know if that's a red flag or not. I know he mastubates, but it's the doing it at work that confuses me. Help..