The Last Tile Finder (LTF) is a reusable tool that helps you find the last 1-10 tiles in a mine. It works anywhere except repeatable mines, but only when there are 10 or fewer tiles remaining.
The LTF is sold in the 'Extras' tab of the shop. After level 30, it can be bought for 250 , or after level 50 it can be bought for 50,000 . Once bought, it's available permanently (but needs time to recharge after use).
Whenever you're in a non-repeatable mine, the LTF button is visible in the corner of the screen.
Clicking the button opens a dialog which will either tell you there are too many tiles left, display the recharge options, or let you activate the LTF.
Once activated, a golden trail will run from Diggy towards the first detected tile, or a door that leads to tiles. When you go through a door (or if you lose sight of the trail), click the LTF button again to show the trail again. The LTF will stay active in that mine until it is cleared. Recharging takes 24 hours from the moment the LTF is activated, and it can be active in multiple mines at once - there's no need to rush to clear them.
Some maps, especially those with looped paths, can confuse the LTF. If it's sending you in circles from door to door, the workaround is to visit each screen of the mine, running the LTF each time - if it leads to a door ignore it and go to the next screen. Eventually you'll be on a screen where it can lead you to tiles. To make sure you visit all the screens, check the wiki's maps for that mine.
If the last tile is somewhere inaccessible, the LTF might not work, or might direct you straight through the obstacle. Try to clear a path by moving stones, pulling levers etc.