who are they?

Well there can be many possibilities. He might have called some insect (like cockroach) treatment professionals to do away with them and they are wearing masks (to save themselves from chemicals), waiting for him ready for operation. they might be the persons in baseball game one who stands behind the player and other the umpire while player is completing a home run. They might be paramedics who came on some neighbor's call when someone from that person's family got a heart attack or some other serious thing. He might be a terrorist and two commandos or special action force personels, whatever you call it, might be waiting for him wearing masks. I can't think of any other situation at the moment, may be after a while if something comes to my mind, i'll add.