People say a lot of stupid things. On the face of it, the phrases above seem contradictory. But sometimes when you take a deeper look at it, there is a
paradox.
In this case, it has to do with the meaning of the word "lost." If you define "lost" as meaning "in an unknown location," then the phrases contradict each other. But more commonly, people use "lost" to mean "irretrievable" or "unable to be found" in the permanent sense. In this case, an object can be "lost" in the sense that the speaker does not know or cannot recall its location, but not "lost" in the sense of being beyond recovery.
I lost my watch. I could not recall where I had left it, but I knew that I had lost it inside my home. So, I could consider it temporarily lost, not permanently lost. If I think of "lost" in the permanent sense, then my watch was not "lost," I just didn't know where it was.