Couple of weeks late, but had same problem here.
Filled mine with the Rain-X stuff a couple of weeks back and almost immediately after the washer fluid light came back on telling me it was low again already. Filled it back up (it wasn't empty to start with) and light still didn't go off.
When I took it in this past Wed. for its 5K mile service, mentioned this to the SA and he immediately goes "Did you happen to fill it up with Rain-X?" I was like WTF.. "Yes". He told me I was the 4th person that day alone with the problem in their Audi.
As he explained it to me, they think there is some waxy substance in the Rain-X fluid that coats the surface of the sensor in the tank and causes it to not be able to 'see' if there is any left, so throws the light thinking it is empty.
They fixed it for free by draining, removing the sensor and cleaning it, and replacing some grommet in the assembly (according to the work order anyways), and refilling with blue stuff. Light was out when I got it back.
As prevalent as this problem seems to be occurring, you'd think Audi, or someone, would put out an advisory or tech bulletin telling owners not to use Rain-X fluid, but oh well. Probably too minor to show up on their radar..
Hope you got it sorted out OP.
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