I have a couple of indoor-outdoor cats.  One is a white cat and the
other is a Siamese cat.  They are both trained to come to me when I
whistle for them.

They are both male cats, and they are both NEUTERED.

The white cat spends about half his time indoors, but the Siamese is
almost completely an outdoor cat.

The white cat ALWAYS comes inside at night.  During the warmer months,
the Siamese NEVER comes in at night, but during the colder months, the
Siamese does come in at night.

Now that it has started to get colder, I whistle for both of the cats
just before I go to bed.  The white cat usually comes running almost
immediately, but there are times when I do NOT see the Siamese cat at
all, and I simply have to leave him outside all night.  Once in a great
while, NEITHER cat comes to me at all, and I have to leave them both
outside.  I hate to leave either one of them outside because I hate the
thought of them being cold all night long, but I have to get to bed and
I can't afford to look for them all night.

MY QUESTION: Both of these cats do NOT go further than 2 houses away
from my house.  I can whistle during the day and they both come running
up to me.  BUT WHAT ARE THEY DOING AT NIGHT THAT WOULD CAUSE THEM TO
IGNORE ME WHEN I CALL THEM?  WHAT ARE SOME POSSIBLE "HIDING" PLACES?
Why would a cat prefer to stay outside in the cold instead of coming
into a warm house?

I am just curious where they are when I can't find them.  I live in a
typical neighborhood setting.