Toriel's Sanctuary is a location guarded by a magical barrier. This barrier can seemingly hide her presence from people, and even gods. Though it's difficult, an individual can locate where it is. As Sans is shown to track it down, and meet with Toriel.
The sanctuary resembles a garden of which has been growing for quite some time. Wildlife is often seen when Sans and Toriel discuss things, as birds are seen. No constructed buildings or objects are seen, and is presumably void of human life.
Sans locates Toriel's sanctuary and destroys the barrier surrounding it. After a short meeting between Sans and Toriel, Sans leaves.