Tasks either need to be assigned to someone or homed in a project, if they do not have one or the other property they would be considered a deleted task,
@lpb – This did behave differently than I expected! I can’t believe I hadn’t come across this/tried this before today!
@Matt5 – to add to what @lpb has said, you can add the filter of assigned to “Nobody” in the advanced search to limit the results you return when searching for a task. Unfortunately there isn’t a way that I am aware of to select “No Projects”