Although you said there are no symptoms, it could be your water parameter or tank set up is not right for them. If they are indeed acceptable, they could be died from other cause.
Such as stress from the trip to your home not long after they had trip to store and hadn't recover from the first trip yet. That is why you have better chance of adopting new fish to your home if the fish has been at the store for a while. So it is always good idea to check/ask how long the fish has been in the store before you make the purchase. Also I would avoid the fish just came in the store. Usually store have certain stock day once or week or twice if the store is busy. I usually find those day and go 1 or 2 days before their stock day. So hopefully the most of the fish has been in the store for at lease 4~5 days. It probably better to be longer but not always work. Also most likely the slimmer selection. But they would be stronger survivers and hopefully they are the one recovering from the long stressful trip.
Also it would be better to get the fish from other local hobbiest. Their fish would be better taken care of as well as less stressful shorter trip and similar water parameter(since most likely they are kept in local tap) thus easier acclimation. After all they love their hobby and less bussiness of store. I sometime get the fish from other aquarium members. They usually have auction and maybe some advertisement and such. And it usually cost less also. They don't have all the fish in the store but they may have something you been looking for. You never know.
Well, this is just my 2 cents. Good luck find the healthy fish.