tim birr wrote:
I gather the hole wasnt there when you built it or you'd have
noticed right?

Wood boring beetles or that Bee type that likes wood. You may
want to treat the area for that before you get more holes if
that is the causitive factor.

There was a knot there where the "hole" is now. It just looks like the
knot fell out of the hole, so to speak --;0

I'm not much of a carpenter, and it's possible the hole was there when
I put it on, but more than likely "fell out" as I was pounding nails.
At any rate, it's not insect or bird damage.

So, again...best fix??

The BEST fix is to look around for the missing piece and glue it back in
place. Next best is to carve a plug from similar material then glue IT into
place.

The usual fix advocated by architects for this and similar "mistakes" is to
plant ivy. Following this strategy, can you cover the hole with something?
Ivy, a bird feeder, faux mailbox, Marilyn Monroe calendar?