Just to throw in a couple of more possibilities, even though the issue in this particular case has been resolved.

I had a squeak a couple of months ago that drove me nuts for a few days until I figured out that it was the edge of my shoe lightly brushing the zip tie that holds the cadence pickup magnet to my crank arm. Moved the cleat over about 1 mm and squeak solved.

Another tip I read just last week was spray a little silicon lubricant on your shoe sole in the area under your cleat (primarily Look and similar cleats). Slight flexing of the sole can cause a squeak against the cleat. This is one that could also be affected in the rain since a little water in there would also provide some lubrication for a while.

FWIW,
Raymond