Prescriptive easements can exist over another's real property and are acquired by continued use without permission of the owner for a period provided by state law to establish the easement. In California it is at least 5 years. The problems with prescriptive easements are that they do not show up on title reports, and the exact location or use of the easement is not always clear, which seems to be the case here.
The question here is whether your neighbors will file litigation in order to preserve their ability to continued use of your stairs. Some courts find that simply using a part of another’s land regularly for a shortcut is enough for a prescriptive easement, while other courts are very reluctant to grant rights on someone else's land and require the use to be substantial. If your neighbors have another way to access their property then you should be able to put them on notice with a letter and a sign that tells them they do not have permission to use the staircase.
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