Although quite rare, it is possible that a Sanctuary disappears from the Map. This can happen when the real-world data gets updated or if a Sanctuary was in a dangerous location that had to be removed.

You can still access that Sanctuary temporarily through the character sheets or through your Alliance list of Shared Sanctuaries, as explained below:

Accessing a Sanctuary Through a Collection:

1. Open your Collection. 
2. Find a creature that was in the Sanctuary. 

Accessing a Sanctuary Through an Alliance:

1. Open your Alliance screen.
2. Tap the Shared Sanctuary button.
3. Find the Sanctuary in the list and Enter it. (Note: This will only work if the sanctuary was recently shared)