I just tested a 3% solution of baking soda dissolved in pure water with a lab grade pH meter, and got a reading of 8.3.
How accurate is your measuring system? Do you have anything to calibrate with that high? Also, you definitely used baking soda, not washing soda, correct?
If it were me, I’d check the rainwater, and rainwater with baking soda dissolved in it, see if you have anything close to the water you tested before. If rainwater and baking soda measure a lot lower, I’d start wondering what other residues might be in my equipment. I wouldn’t try to drink it, but by itself, pH 10-11 will just dry out your skin, it won’t burn. However, I might worry that whatever is increasing the pH is accompanied by some other chemicals, especially if my equipment was used.