I have experienced this on numerous occasions. My freshly sanded wood gets a drop or two of rain on it. I let it dry. I try staining it and wow! The stain takes up different. Yesterday I had to do an emergency wet sanding of a piece that got a few drops of water on it from my raincoat a week earlier. It was red alder and the spots took up three times as much bartleys stain as the dried areas.
I am mainly curious as to what is physically going on. I know now to cover my wood. I am very lucky that alder does not exihibit the photo-sensitivity that some woods do. I use to have to cover my unstained pieces with black plastic. That was a pain.
Thanks
Tor
ps. I am guessing that the wood pores simply expand, thereby accepting more stain. I wonder if anyone uses this technique to deepen the uptake of stain...