I think this is the most likely explanation for the teabag. Someone was drinking water, someone else was drinking warm tea. Someone (an adult I think) set the bag in the empty glass, rinsed out their mug and probably put it in the dishwasher. I wonder if there are any pictures to see if there were dishes in the sink.
Anna, if that was the explanation you would think that one of those people in that house that day would have stepped forward and said "hey guys, you know I had a cup of tea that day". And who knows, maybe the told John or Patsy who in turn told their lawyers who then said "never mention it again". It wouldn't surprise me in the least as my faith in people has been greatly tested while researching the people involved in this case.