I assumed the bottom of the tank was visible under the stand but I am guessing you have a solid top stand by your comment above.
You could try placing a paper towel or tissue near the tank bottom and see if it wicks up any water. Maybe you had a water spill that leaked down the outside of the tank and some of the water pooled under the tank. This may account for the problem you are seeing.
If you have a portable fan, maybe direct it toward the bottom of the tank where it meets the stand/cabinet and see if the marks go away.
Right... just been underneath... there is paper stuck to the tank... it is peeling off and damp on the end with the stripes (the stripes being the glue from the paper) so... could it be a leak? Or what? My water levels have been pretty constant so I'm not certain on the leak hypothesis... any ideas?
No clue, id dry it off well and check it in an hour and see if anything is wet. Only other thing could be condensation But you said your tank is warmer than the air
It looked a lot less strange when I left this morning... I pointed a fan at it overnight and mopped up assent standing water. Just hoping I don't come back on Sunday to a broken tank... I've marked on the water level currently and I'll see what it's like on Sunday, and go from there...