my budgies died

Question
I have had my one male and one female budgies for seven years.  I was told they were immature when I got them.  I just found them both dead in the bottom of their cage and am trying to understand what happened.  It can't bring them back, but perhaps I can save someone else's birds.  These are my suspicions:  Hartz millet sprays for Christmas.  An either fir or hemlock Christmas tree which they ate.  A new baby guinea pig in the room with them which came from a supplier who also has birds.  Fumes from wood stove and propane indoor grill and burning food in oven.  Kaytee parakeet food.  I've always previously fed them a vita mix for parakeets from our local pet store.  We have a cat, but they were simply lying on their cage floor unruffled.  I saw no blood or ruffled feathers.  Also, they were alive when we left this morning and we took the cat out with us. They had plenty of water.  They were beloved.  What do you think was the most likely cause?

Answer
Oh my God, I'm so sorry to hear about your loss...I just lost one of mine Christmas Eve.

The millet sprays are unlikely to have caused it.  If you're worried about it though, call Hartz and ask them if they use any sort of pesticides on their millet sprays.
Some firs trees can be poisonous to budgies, but you would have seen symptoms.
The guinea pig probably wasn't the problem.  It's not impossible, but it's unlikely that he transfered pathogens from the supplier's birds, brought them home, and infected your birds.
The most likely culprit was either the stove or the grill.  Birds are extremely sensitive to the gases emitted from these sort of appliances.  It would explain why they both suddenly died without symptoms or signs of illness.  I would take this as a warning to open up the windows and vent out the house.  If the gases hurt your birds, they could hurt your family or your other pets.