I've been using my GS3 for 7 years and really enjoy how well it works. It's been one of my most best purchases ever. But the one feature I'd love to see is a warning light when the water reservoir is low. It requires removing the drip tray to check water level, and often it's something I forget to do. So, looking for a simple probe and light to go on when the tank needs refilling. I could probably build something, but wanted to check first if there is such a device. Yes, I know, I should plumb it to my water line, but my wife wont tolerate a hole in the counter.

Try Googling "liquid level sensor" or "float switch". Maybe add the words "mini", "miniature" or "small". Looks like there may be models that might fit in the tank, but you'll have to drill some holes for mounting the sensor and routing the wires out of the tank. I'm thinking that suspending the float from the top cover and routing the wires out a hole in the cover might be the best approach because there's little if any clearance around the front, back and sides, and you don't want to have to waterproof the holes. Depending on what comes with the level sensor, you may need a small battery-driven LED circuit for the indicator. You might be able to find something off the shelf for that. If not, it's easy to build one.

Here's an intriguing solution for an Arduino-based non-contact level sensor. Given the low cost and the many Arduino boards I have lying around, I'd try this one, if only for fun. However, I think the lack of clearance around the reservoir probably rules it out.

A much simpler alternative is to get in the habit of topping up the reservoir as part of the cleanup ritual after every session. Or before every session.

Me, I drilled a hole in the countertop. Can't begin to tell you how much plumbing-in has increased my enjoyment of the GS/3. I'll never go back to the reservoir. My wife is very understanding about "letting" me do what makes me happy, as long as it doesn't impinge on her happiness. The hole is hidden under the machine, so she doesn't see it, and my wife is content to let me worry about what to do about the hole (cover, patch, install an auxiliary sink/faucet, nothing), if and when we sell our house. I might have hesitated more if the countertop was granite or marble (a huge slab of which we have on our island), but it's Corian. Easy to patch, and I wouldn't be surprised if a house buyer would want to upgrade the countertop to stone anyway.