refresh makes "unknown" threads disappear

Hello,

All the threads with the Unknown status disappear from the threads pane
whenever I make a refresh. Can I be sure that they are no longer taking up
resources in the JVM ? I mean, can I be sure that all threads in  the VM are
those actually showing up in the pane ? I'm using the debugger to check this
kind of memory leaks. would anyone advise against this practise or recomend
a better approach ? 

Thanks.
Nuno