I had a hunch, after I got home I checked, and yes: some birds built a nest on top of it. The top of the thing is may 1x2", so I admire their engineering, but it renders the thermometer pretty useless for the rest of the season. I think it's some kind of titmouse or something else small, brown and crested. When we first moved in, there was a nest on top of the junction box for one of the outside lights, last year there was one on the bottom flange of the I beam that holds up the ceiling of the garage. They seem to like places with close top cover -- as who would not, I guess, in a climate where spring is full of wind and rain.
Next year, I think I might put up some ledges under that same balcony, and some hardware cloth around the sensor. (This is the second one we've had incapacitated by wildlife; the first had airholes that were apparently just the right size for nesting earwigs.)