Yeah I am at 33.5 ppm. I am aiming for 35 but as you said that does not explain such drop in kH..Check your salinity, something is not right, most likely a testing error or salinity is off, but your salinity would have to be crazy low to be 4 dkh off.
I think the evaporation will be less than you think, my tank is 14g with a mesh lid, maybe 500ml evaporation a day. Boiling the kettle or taking a bath/shower would put much more moisture into the room than a small tank.