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Ghost object

I have just lost an hour and a half trying to find a heap leak in my
code. It is a simple object graphics, universal replacement for PLOT but
with zoom, edit etc. capabilities. Anyway, once I kill the plot window,
a cleanup routine is called that does total sweep of the heap variables
by looping through object fields and destroying objects and pointers.
Once its done, I'd call our favorite Heap_gc to see if there was a leak.
Sure enough, it was always finding and deleting an instance of
IDLGRFONT. I tried everything I could: commented out the only line where
IDLGRFONT was used, tried all sorts of explicit object destruction
immediately after creation, etc. I felt it was not my fault, so I tried
"Clear IDL" and .Reset, and .Full_reset. Same thing: once I recompile,
run and kill the object tree, IDLGRFONT would still always be found by
Heap_gc. It was not a fake, as its heap ID would be always higher each time.
Frustrated, I quit IDL and restarted. What do you think? It stopped
doing that. I swear the code is the same, because I went through every
single line checking it, saved it, tried it, got a leak and quit IDL.
But after restart IDLGRFONT stopped appearing in the heap.
I have no idea what it was, but it certainly complicated my leak search.
Does anybody have an explanation?