I have kept mine premixed for weeks and weeks inside the standard blue plastic bottles, so that smell may be some leachate from the plastic itself which also may be unharmful, where switching to other bottles may remove it, or it could be the gaseous form of items that precipitated or changed form upon mixing into solution and then when you put the lid on it all of it was held stagnantly until you refreshed it. I'm sure if you test your buckets by keeping one sealed and then one vented from time to time you'll know, there's no reason for that smell to keep coming back after being vented a few times, that's plastic leaching if that happens.
Cool you are from Douglas AZ. In the 90's we tore into that place, the police department of AP border zone is a mighty scary place to get into legal trouble, I really miss all that fun stuff./ we were from playas nm so douglas was like going to the big city for us country kids in about 91
