The hatchlings set the standard, not the adults
Mourning geckos are small, social, and entirely female, reproducing by parthenogenesis, which means a settled colony quietly produces eggs and hatchlings without any change the keeper planned. Those hatchlings are the escape benchmark. An adult may respect a door seam, but a baby mourning gecko can pass through a gap beside a hinge, a vent slot, or the corner of a lid that an adult could never use, and it will explore all of them at night.
This is why so many keepers find a young mourning gecko living behind the enclosure, in a houseplant, or in another pet's vivarium nearby. The colony did not break out by force; it simply outgrew the keeper's assumptions about what counted as closed.
Sealing doors, vents, and corners for babies
Run a strip of soft foam weather seal or a bead of cured aquarium-safe silicone along the edges where the door meets the frame, so the closed door compresses a continuous seal with no tapered slot at the corners. Do the same where a lid meets the walls, and inspect the hinge side, which is the seam keepers most often forget because the door itself hides it when shut.
Cover every ventilation slot and drilled hole with mesh fine enough to stop a hatchling, fixed on the inside face so a gecko cannot push past its edge. Route cables through the smallest workable holes and seal around them. Then test the whole enclosure in a darkened room with a torch inside or outside it: any line of light escaping a seam marks a route a baby can investigate, and each one needs closing before eggs in the colony begin to hatch.
Eggs, plants, and hidden routes out
Mourning geckos glue their eggs to surfaces, often in a bamboo tube, behind a background, or on the underside of a broad leaf. Those positions matter for containment because a hatchling's first journey starts wherever the egg was laid, sometimes already beyond a loose background panel. Check behind backgrounds and inside hollow decor during your sealing work, and close the gap between a background's top edge and the lid, a favourite hidden corridor in planted setups.
Plants with stems passing through a vent or a door kept ajar for a trailing leaf quietly defeat good sealing. Keep all growth inside the sealed volume, and when you remove a plant for trimming, inspect it at the enclosure, since a baby gecko riding a leaf out is a common and completely silent escape route.
Counting a colony and living with the occasional escapee
Exact counts are hard in a colony, so build habits that do not depend on them: open one door at a time, scan the door's inner face and rim before opening fully, and work over a clear surface where a dropped or darting gecko can be seen. Keep a small ventilated tub within reach so a gecko found somewhere unexpected can be secured before you investigate how it travelled.
If you find a gecko loose, check the warm, humid places nearby first, including the backs of neighbouring enclosures and the soil surface of houseplants, then re-run the torch test on the home enclosure and seal the seam you missed. A single found baby usually means the route is still open for the next one.
Because hatchlings can slip out through a seam nobody spotted, KinsLe's Lost Pet Alert System can alert other pet lovers and let a finder message you directly about a gecko found outside its home.