Search the edges before you search the middle
Tortoises travelling through a garden behave like they are following a handrail: they track along fence lines, walls, hedge bottoms and the edges of paths, turning corners rather than crossing open ground. So start at the boundary and walk it slowly, checking every gap at the base of the fence where a determined tortoise could dig or squeeze through. Look behind planters, along the shaded side of walls and under the lips of raised beds. The wide open lawn that dominates your view is the least likely place to find the animal, yet it is where most people spend their searching, which is why lost tortoises so often turn up in the corner nobody checked.
Get down to tortoise height and look for dug-in shapes
A settled tortoise is astonishingly hard to see from standing height: the shell reads as a stone, a clod of soil or a shadow under leaves. Work the promising areas on your hands and knees, parting ground cover gently and scanning for the curve of a shell or a patch of disturbed earth. Check leaf litter, compost heaps, mulch, log piles and the dry sheltered ground under sheds, benches, trampolines and dense shrubs, because a tortoise that feels exposed digs itself down until only the top of the shell shows, and sometimes not even that. Run your hands lightly over suspicious mounds rather than stabbing with tools, since the animal may be just beneath the surface.
Let the tortoise's routine lead the second search
If the first sweep fails, search again on the tortoise's schedule rather than yours. On the first sunny morning after the loss, check every sun-trap the animal favoured: the patch of paving that warms early, the south-facing wall base, the bare earth spot it used for basking. Set out favourite foods, a shallow dish of water and, if you have one, the tortoise's own shelter or hide in likely areas, and check them quietly every hour or so; a hungry tortoise that will not come to your voice will still come to lunch. Avoid strimming, mowing or moving heavy items in unchecked areas until the search is done, for the animal's sake.
When the garden has given up its answer, widen the search
If two thorough searches find nothing, assume the boundary failed and the tortoise is out: check the other side of every fence section, ask neighbours to search their gardens the same low, edge-first way, and look along the warm edges of pavements and driveways nearby. Tortoises move slowly but steadily, and a head start of a day still leaves them within a few gardens in most cases. Beyond the garden, KinsLe's Lost Pet Alert System holds the alert at no cost in the tortoise's own profile, places it where map readers will notice it, and lets a spotter reach you directly instead of trying to catch the animal.