I knew that the jewels were valued at several thousands of dollars.

If I went away some one else might come into the room and possibly steal the jewels, for they were lying in plain sight and were valuable enough to tempt a weak-willed person.

I sounded an alarm and stayed in the doorway.

I refused to leave the room until Mrs.Hubbard returned and counted her valuables.
She found them all there and thanked me for guarding them.

She said it was by an oversight that she had gone away without locking up her treasures.