So, I discovered that the Prius does not warn you when the wiper fluid reserver is low. Or does it, in a way that is so inconspicuous that I did not see?
Well, it wouldn't be a bad idea to pop the hood once in a while and check the dipstick for your oil level...you can check your washer fluid at the same time.
One of the Prius "POSITIVES" is the FULL GALLON reservoir for windshield wiper fluid. In freezing weather, I use a generic, auto-parts-store anti-freeze solution. I've never felt the need of a gauge or indicator for fluid level. I like the suggestion of taking a look 'under the hood' occasionally to check around including ICE oil level, windshield wiper fluid level, etc. You might even find that a squirrel or possum has established a nest under there...
The hood latch is under the dash, left side of the steering column. Just pull it, the hood will open, and the fluid bottle is on the left.
