Methanol evaporating from the mix?

[JohnA]

I find myself having to top up the W.I. bottle even when I haven't used the car. It could be 50cc after a week of inactivity (garaged).
I'm using the stock windscreen washer bottle, and the water tube feeding the aquamist pump has a tight-fitting grommet and silicon as well.

I suspect that it's the methanol content evaporating, but it could just be my paranoia. :?

Anyone else have the same problem?