How did Holmes know there would be a ventilator in the room in "The Adventure of the Speckled Band?"

I would argue that Holmes did not know there would be a
ventilator in the room when he first started to formulate his idea about what had
happened to Julia Stoner.  Before he comes to Stoke Moran, Holmes has already decided
that something has been smuggled into Julia's room to kill her.  But he does not know
how that could have been achieved.  We know this because he examines the outside of
Julia's room carefully.  He even tugs on the shutters and examines them with his
magnifying glass.  At that point,

readability="8">

"Hum!" said he, scratching his chin in some
perplexity, "my theory certainly presents some difficulties. No one could pass these
shutters if they were
bolted.

Clearly, at that
point, Holmes did not know about the ventilator.  He had thought there would be some
more obvious way of getting something in to the room.  He then goes in to the room and
examines it all very carefully.  Only after a long time does he notice the ventilator
and even then, he only notices it because the rope for the bell-pull is attached to
something near the ventilator.

So, it is not as if Holmes
knew in advance that there would be a ventilator there.  Instead, he only found out
about that ventilator after doing a great deal of searching.