As a current pumper, I think you should definitely explain what the issue was because otherwise he'll wonder what you were REALLY doing.
Of course, I'm not to secretive myself. I put a sign on my door three times a day that says "PUMPING, DO NOT ENTER" and very loudly declare the pump when I fly. In fact, when a screener told me he might have to confiscate my cold pack because I didn't have any milk in the bottles (was going to pump AFTER security), I was ready to announce "I'll fill the bottles right now." But I didn't have to, luckily.
