Question
Since I last spoke to you, we had the plumber ream out part of our pipe due to an overflow.In addition to reaming out a certain toilet, he discovered a slow leak around the base and resealed it. That could have been where they were coming from. That was last week. As far as the plants in one room with the door closed, I could not find any evidence of flies. I have since then, put the flowers back where they were. Two days ago, I only found one fly which landed on the computer screen. Every so often, we apply the green gel we get through a web site. I'd appreciate a follow up from you as to whether we have a handle on the problem.  

Answer
Sounds like your are on the right track. A leaky pipe or toilet by itself won't produce drain flies but if the leak causes algae ("slime") to grow then drain flies can develop. If you see any more flies it might still be a good idea to capture a few and get them identified through your county Cooperative Extension office, then you'll know for sure.

Jack DeAngelis
http://www.livingwithbugs.com
http://livingwithbugs.blogspot.com