Chuckle. I have a similiar problem on a 7 year old house.

This is what the previous owner told me during the tour:
-
The light at the top of the stairs is a 3-way; it has a switch at the
bottom of the stairs, and one at the top of the stairs. There is a third
switch at the end of the hall to switch off the bottom recepticles in
the upper hallway (for small lamps, etc.
-

About 3 months ago the ceiling light at the top of the stairs wouldn't
switch on no matter which of the switches used: the one at the bottom of
the stairs nor the one at the top. Dead as a doornail.

After tinkering, I discovered that the 3rd switch in the hall 'for the
recepticles' actually cut power completely to the ceiling light and both
of it's 3-way switches at the top and bottom of the stairs. Once you
flip the 3rd switch, neither of the other two switchs can turn the light on.

Not only that, but it certainly didn't control any of the recepticles in
the upper hall; it simple cut the power to the ceiling light and it's
other two switches.

Go figure.
:-)