Ignoring the fact that my new apartment here in Japan is like a god damn fortress despite it also
obviously, by its facade, not being the apartment of
a particularly well-to-do person so no intruder is likely to come in, I would probably do what I did the last time I thought
an intruder had gotten into my previous apartment (I had just bought a dog, and so I wasn't entirely sure if the sound was
coming from the dog or something else because it was pretty "non-doggy" sounding)
Very quietly get out of bed, readied my cell-phone to quick-dial the emergency number, took out the baton I keep under the bed
next to the night-stand, and positioned myself by the sliding door to my bedroom, on the side
least likely to be the one a person unfamiliar with my apartment would look to first when opening the door,
and then I stayed there until the sound quieted down.
Once I waited for a reasonable amount of time, I got my tac-light, and did a sweep of the apartment, checked all closets, windows
and the front door, and finally got back to sleep.