Every time you wash a car with a blocked drain it will fill up a bit more, who needs rain...
Trouble is the motor lives in a little container (soundproofing), above a big plastic moulding under the roof fabric. The moulding has the drain hole.
If it gets blocked, the moulding will fill up enough to then overspill into the little container (which is not sealed adequately and doesn't have a drain hole)
Even if you clear the moulding drain hole afterwards, it won't empty the little container, and the motor will sit immersed in water, rusting away until it dies. No amount of drain checks will empty that water away. The damage will be done just waiting for the time bomb to kick in.
You just have to hope it has never reached that level, and remains clear afterwards.
If you can do it yourself, (its pretty straightforward) a new motor is £300, big whoosh. 2 tyres are more than that.
My drains were cleared when I first bought my previous Z4, but 8 months later I emptied a cup of rusty water out of the soundproofing after the motor died.