The mourning gecko is the escape artist other gecko keepers warn you about — not because it is strong, but because it is tiny, flat, nocturnal, and endlessly productive. An adult can pass through a gap you would swear was sealed, and a hatchling is smaller than the adult's head. Security for this species is a sealing problem and a population problem at the same time, and both need solving before the vivarium is stocked, not after the first gecko appears on the outside of the glass.
Seal for the hatchlings, not the adults
Go around the finished vivarium with a torch and a critical eye. Front-opening doors often leave a slit where the two panels meet or hinge; line it with a snug strip of foam or silicone edging. Cover vents with mesh fine enough that a hatchling cannot pass — standard insect screen can be too coarse for a newborn mourning gecko. Block cable holes for probes, lights, and misting lines with mesh and sealant, and check the lid and door frames after every deep clean, because seals shift as doors are opened hundreds of times.
Assume any gap you can slide a fingernail into is a door. Keep the vivarium itself in a room where an escapee is recoverable: door closed during maintenance, drains and overflow holes covered, and no easy route into wall cavities or behind fixed appliances.
Maintenance habits for an animal you cannot catch by hand
Open doors slowly and look at the door edges first, because mourning geckos love to sit exactly where a swinging panel will sweep them out. Work over a wide tray or an empty bathtub for plant trimming and egg-spotting, so a jumper lands somewhere visible. Never try to grab one — at this size, fingers are a threat and a fall risk; coax the gecko onto a leaf or into a small ventilated pot instead.
Count heads at feeding time under a torch after lights-out, when the colony comes out to hunt and lick food. Regular counts are how you learn the colony's real size, spot a sudden absence that suggests an escape route, and keep track of the eggs glued in hidden corners — eggs this species sticks to wood, plant axils, and the undersides of leaves, where they will hatch in place whether you planned for them or not.
The population is part of the safety plan
Mourning geckos are parthenogenetic: every animal is female and fertile, so a pair becomes a colony and a colony becomes a crowd. Overcrowding is a safety issue — competition at feeding stations, stress, and a rising temptation for keepers to make bad decisions. Plan homes for offspring with other keepers before you need them, track how many juveniles you have moved on, and never, under any circumstances, release surplus geckos outside. A released mourning gecko is an invasive species problem in warm climates and a death sentence in cold ones; both outcomes are the keeper's responsibility to prevent.
If a gecko does vanish, check warm, humid hiding spots near the vivarium at night, set a tiny feeding station with fruit food nearby, and be patient — they often stay close. If one turns up beyond your home, KinsLe's Lost Pet Alert System can send a free alert to nearby Circle of Love members, and a finder can message you directly to get it back.