Smell, not memory, drives the first reaction
Cats recognise each other largely by a shared household scent built up through sleeping in the same places, grooming, and rubbing against the same furniture. A cat who has been missing for days comes home carrying a completely different scent picture: outdoor smells, other cats, a finder's house or car, disinfectant from a vet visit, and stress itself. To the housemate cats, she may register as an unfamiliar cat wearing a familiar face, and hissing, growling, fleeing, or fixated staring are their way of treating her as a stranger until her scent settles back. The returned cat, meanwhile, may hiss first — she is exhausted, possibly in pain from a minor injury, and primed to expect threats — so friction can start from either side without any real change in how they feel about each other underneath.
Start with separation, even if they were close before
However bonded the cats were before she went missing, give the returner her own room for at least the first day or two, with her own litter tray, water, food, and bedding. This protects her rest, lets you monitor her eating and toileting after her time outside, and prevents a midnight confrontation when you cannot intervene. Feed the cats on opposite sides of the closed door so each associates the other's new scent with a good event, and swap bedding daily — rub a cloth gently on the returner's cheeks and place it near the housemates, and bring her a blanket the housemates have slept on. If there is no tension at the door after a couple of days, that is your signal to move to the next step.
Short, supervised meetings built around food
Open the process gradually: let the cats see each other through a baby gate, a cracked door, or a carrier before they share open floor. Keep first face-to-face sessions brief, in a neutral room with more than one exit, and time them around meals or a favourite food placed well apart so nobody has to compete. Watch body language rather than hoping for instant affection — relaxed eating, turning away voluntarily, and ordinary blinking are good signs, while fixed staring, a thrashing tail, flattened ears, or one cat blocking the other's route to the door mean the session should end calmly. Separate without scolding, return to scent swapping for another day, and try again; most pairs rebuild their old ease over a week or so of these small, successful meetings.
When the strange behaviour needs a closer look
Some post-return friction is not about scent at all. A returned cat who hides from everyone, stops eating, cries when picked up, or guards one side of her body may be hurting from a fall, a bite wound hidden under the coat, or an abscess forming out of sight, and pain makes any cat short-tempered with housemates. A housemate who was fine at first but becomes withdrawn days later may simply be stressed by the changed routine. If hostility is intense, one-sided, or still escalating after a week of careful reintroduction, or if any cat stops eating for a day, book a vet check for the returner first — and sooner if you find swelling, heat in the skin, limping, or a feverish, quiet manner.
Keeping the whole household on the same plan
Reintroduction fails most often when different people improvise — one person lets the cats together for a cuddle while another is still doing door feedings. Agreeing the stages once and sharing them through KinsLe Family Sharing means every family member sees the same separation, feeding, and meeting notes for the returned cat, so she gets one consistent reset instead of a different rule at every door.