EMPTY BRAKE FLUID RESERVOIR for 3K miles

What could go wrong if I drive 3K miles without brake fluid?

I took my 2017 Prius to a Toyota Service Department (TSD) for a brake fluid flush. Little did I know, TSD forgot to put the cap back on the reservoir. (Today) 3K miles later, I popped the hood to check my dipsick, when I noticed the cap was removed and kept in a place foreign from the reservoir (pictures below). Concerned, I inspected the reservoir and found it EMPTY. BONE DRY. That's when I realized my brakes had been grinding since leaving TSD. Now I am VERY concerned that there is damage to my brake system -- master cylinder, brake lines, calipers, pads/rotors -- caused by insufficient fluid for the last 3K miles. What if there's corrosion in the brake lines, now? A failed master cylinder? Uneven wear to my pads and rotors? My calipers? Etc.
I know that insufficient fluid levels can introduce air and moisture to your brake lines, causing corrosion and brake system failure, which can be extremely expensive to fix. I want to gather as much information possible and use it to convince TSD to fix the problem.

ALSO. I just bought the Prius (used) 7K miles ago from Tom Wood Lexus (used with 81K ODO miles). Tom Wood replaced the pads and rotors with brand-new OEM parts, a $2,283.84 job, which I got for free. However, I am afraid that this TSD mistake has unnecessarily accelerated the wear of my brakes, reducing their value and utility. And worse, my safety. (They are grinding, after all.)

 

Thank you, @FuelMiser. Really, my priority is addressing the fluid problem. So your advice is helpful. While I'm at it, I'll have the whole system inspected. My next concern is, TSD will deny it and not fix it fully or properly. But it's an imperfect world, and I have to prioritize the maintenance of my Prius over the cost-advantage of determining liability of the problem.

I should clarify that it's a light rubbing sound, which I am not certain is related to the fluid matter. I wonder if it's the sound of rust being rubbed off by the pads during short trips, or something more serious.

 

I'll clarify, what I hear from braking doesn't sound like a dramatic grind. Rather, it is a rough sound; not smooth. So I long ignored it because I assumed it was typical rotor-rust being rubbed off by the pads. But now, after discovering the empty reservoir, I am more concerned. Which isn't to say it's directly connected to the empty reservoir but cause for concern/suspicion.

Another thing, I can't confirm whether the brakes feel abnormally* soft/spongy when I depress the pedal. BUT I do clearly remember that AS SOON AS I LEFT the service department, MOMENTS AFTER IT WAS FLUSHED, the brake pedal felt more resistance than usual, which I took for a good sign -- I suspected that, since the brake fluid was newly flushed, the brake system was rejuvenated, hence a slightly-stiff, resistant pedal feeling. But within a day or two, I noticed a regression in the feeling -- the brake pedal no longer felt slightly stiff; it decreased in resistance; it got softer. I wondered why it so quickly regressed to a softer feeling. But again, I assumed it was normal and ignored it since. Until NOW.

Now, after discovering the empty reservoir, I am reminded of how quickly the feeling of the brake pedal regressed. So NOW my suspicion is that, the capless reservoir caused fluid to splash out while driving within a day or two after the service. And that, consequently, the pedal softened because of lower fluid levels and, potentially, because of air drawn into the system, which could even suggest moisture followed along, too, concerning me further.

 

I can't confirm whether the brakes feel abnormally* soft/spongy when I depress the pedal. BUT I do clearly remember that AS SOON AS I LEFT the service department, MOMENTS AFTER IT WAS FLUSHED, the brake pedal felt more resistance than usual, which I took for a good sign -- I suspected that, since the brake fluid was newly flushed, the brake system was rejuvenated, hence a slightly-stiff, resistant pedal feeling. But within a day or two, I noticed a regression in the feeling -- the brake pedal no longer felt slightly stiff; it decreased in resistance; it got softer. I wondered why it so quickly regressed to a softer feeling. But again, I assumed it was normal and ignored it since. Until NOW.

Now, after discovering the empty reservoir, I am reminded of how quickly the feeling of the brake pedal regressed. So NOW my suspicion is that, the capless reservoir caused fluid to splash out while driving within a day or two after the service. And that, consequently, the pedal softened because of lower fluid levels and, potentially, because of air drawn into the system, which could even suggest moisture followed along, too, concerning me further.

 

Practical advice. Thanks.

And no; no warning lights. So that's a good sign.

 

RESOLVED! Ed Martin Toyota accepted responsibility for (what turned out to be a pretty minor) mistake and more than made up for it by fully inspecting the brake system while addressing the brake fluid and brake noise, all at no charge. The service manager assigned my Prius to the shop foreman who did an outstanding job in addressing the problem.

Summary
BRAKE FLUID -- tested, replaced, and bled the brake fluid; confirmed enough fluid remained and that no damage occurred; replaced fluid, anyway, for peace of mind.
BRAKE NOISE -- adjusted brakes to reduce noise; determined pads and rotors were after-market and not perfectly to spec which caused rubbing noise; Tom Wood Lexus installed said brakes in December 2023 before selling it to me.

Background
I called the Ed Martin service manager yesterday and presented the problem. We arranged a 7 a.m. service appointment, to be administered by the shop foreman, "our best guy." Also, it rained most of the day, yesterday. Late last night, I drove about 10 miles to test the brakes. The noise was pretty intense at first, but, the last couple miles, the noise was less tense. So, I suspected the intensity of the noise is explained by surface rust and/or moisture being scrubbed by the pads; it rained earlier that day, after all, which I think caused new surface rust to develop along the rotors. Knowing now that the pads/rotors are cheap after-market quality, I have a fuller picture of why I've been experiencing brake noise and expect the noise to persist until I replace them later, in the distant future. Although the pads/rotors are unfortunately after-market, they have plenty of life left to spare; so I will just tolerate the noise until they retire, and then I will replace them with TOYOTA OEM brakes.

To correct my original post: I falsely assumed Tom Wood Lexus fitted my Prius with OEM brakes, which, when selling to me, they claimed was a $2,283.84 value job (insert eye-roll). I trusted that Tom Wood Lexus would provide a higher quality product and thus assumed they'd fit factory OEM parts, always. But my trust was naive. Like any seller, they went cheap to maximize profit.