I left my opened cat wet food in a ziplock bag on a shelf overnight. Is it safe to feed it to the 3 months old kitten? I gave him some the morning after. It didn’t seem to have a strong smell and the cat seems to be fine and playful. I put the can in the fridge now but wonder if it will be okay to keep feeding him it.
Assume the food is unsafe, and dispose of it.
While experts are not agreed on how long an opened can of cat food can be left out unrefrigerated, pretty much all of them seem to say this time is less than the amount of time it was surely left out if it was left overnight. For example, Whiskas (the cat food brand) has written on their website "a maximum of four hours." Wellness pet food writes, "one to two hours." Purina (another pet food manufacturer) says, "canned food cannot sit out for more than a half hour to an hour."
Furthermore, you should not rely on your cat's nose to tell whether the food is spoiled or not. Though their noses are more sensitive than ours, it is still possible for there to be enough bacteria to make them sick without there being enough for it to smell so bad they won't eat it.
Unless the room temperature was very warm, it should be okay. As far as I know pet food cans are produced following the same hygiene standards as human food cans. Meaning the content is pretty much sterile. At the moment you open the can the food gets into contact with the usual amount of bacteria and mould spores that always surround us in the air. That's when the clock starts ticking.
Bacteria can reproduce at insane speeds in warm and moist environments (30 - 40°C) and can probably spoil a can of food over night, but the colder the environment is, the more they are slowed down. So it's possible the food is still good now, but will spoil a day or twoo earlier than if it was kept in the fridge for the entire time.
Mold always takes a little longer (probably 2 - 3 days) to visibly grow. Since mold is a fungus, it grows its roots (called mycelium) through the food first and only then starts growing its fruits (the fuzzy bits) on the outside. So there might be small amounts of mold in the food before you can see them.
Cats are notoriously picky eaters. They have very good noses and usually only eat fresh prey, no carrion or other "old" food. So you should first look for signs of spoilage in the food yourself (like excessive liquid or discolotation), but also trust your cat. If he refuses to eat the food, you should assume that it's too spoiled for his sensitive nose and offer him a fresh can.