As a man with breasts (gynaecomastia) I have had several occasions of nipple discharge. The first time was quit a shock.
I awoke one night with a full, heavy feeling in my breasts and nipples. Not unpleasant, and no pain. I touched my breasts and my hand was wet. I thought it was blood. I didn't want to wake my wife, so I got up and went to the bathroom, careful not to touch things. It was a clear, somewhat sticky liquid without specific scent.
A few days later I had a sharp stingy pain in my left breast, that returned every two,three hours. Breastfeeding rythm says my wife. A cyst said my doctor, but it was clearly a blocked milkduct. I could feel three swollen ducts and one, in the middle, painful on touching. After three weeks everything seemed normal again.
It has happened more times as I notice leakmarks in my pyamas and sometimes I can sqeeze liquid from my nipples. There seems to be no connection with my night sweats and hot flushes.