Update: As mentioned I came home two days ago to two bathrooms full of flies, I prob. spent an hour chasing and killing all that I could see. As mentioned decided to get serious with this, taped up the drain that I suspected and a couple of other drains that I thought what the hell on. Since now two days have passed, I have seen prob. 12-15 flies, pulled tape and zero flies stuck to it, they DID NOT come up and out of these drains. I have noticed ( been dealing with these things so long ) that these SOB come in spurts, I removed the tape and ran lots of water down the drains, I am thinking this will re-feed the nests with the necessary moisture to give there breeding another jump start. Just my observation and opinion is that when I see these large numbers of flies ( like I did the other day ) it is because a whole nest of them has opened. The 12-15 that I have seen the last two days may have been stragglers that I did not initially kill or these things may have another escape route and my thoughts of them coming from this bath drain may have been wrong all along. If I have been wrong all along than why is it they will typically go away for sometimes days at a time after I pour the foaming bath drain unstopper down the drain? Strange. Regardless I have now taped up what I believe is every conceivable escape route meaning all drains in both bathrooms, underneath both toilets and an access doorway that separates the two baths via a sliding hidden door. I am thinking that ( as mentioned ) now that I have run more water down the drain I will come home this evening to find one of these taped areas just riddled with flies. My next thought if I see still no flies will be that the flies will not come to an opening and try and escape where they feel no airflow, I will punch tiny holes in the tape if that is the case ( with a needle ) and see if that makes any difference. BTW been meaning to mention, we have a clean house, we are clean people, cleaner than most in my opinion, this problem is not due to some slobs that keep an untidy home.