It's been at least four months since I sanded the floor/shelf of my bay
window down to bare wood and applied several coats of Minwax Fast-Drying
Polyurethane, clear semi-gloss. Just recently it started emitting an
unpleasant odor.

It faces south, so when there are no clouds, it gets direct sun for the
full day (no shadows from trees). The odor is less intense in the
morning, then seems to build up. At first, I thought it was from a new
poinsettia and Christmas cactus I placed on the shelf. So I removed all
plants from the shelf and moved them to a different room. That room did
NOT develop the same odor, but the bay window room still had it.
Clearly, the plants are not the source.

Putting my nose right next to the bay window shelf, I can detect the odor.

I'm in central New Jersey, so the shelf's bottom is exposed to very cold
outside temperatures. (Of course, there is insulation under the shelf.)
But up until when I went to Florida on Feb 3, this was not a problem.
While I was away, I set the thermostat to 55 degrees. When I'm home, the
lowest I ever set it is 66 degrees.

Why, after all these months, is it emitting an odor? Do you think that
applying a heavy paste wax would solve the problem? Other suggestions?

Thanks,

R1