PRAISE REPORT. It's Friday today. The Pest Control guy came, showed me the nest which was indeed within my kitchen behind the washing-machine, and treated it. It should be ok to enter the kitchen in a while and open my windows at least by tomorrow. Hopefully they won't need a second treatment but he advised to call if they are still buzzing around. Apparently, when the Queen eventually leaves, a dead nest acts as a (free) deterrent against any other wasps colonizing in the same place, so I don't need to call Maintenance to block the hole from the inside (and incur further charges). I'll keep an eye on it over the next week or so, but hopefully the problem has been permanently resolved. Thank you Lord.