Sorry for the long text...
I should have noticed the signs when the guest booked my humble place for 5 days (I rent 3 rooms in the same place with shared kitchen/bathroom/living room). In his first message he said "Dear Rene', I am a neuroscientist visiting Montreal". Then I went to check his profile and he described himself as a Research Scientist (PhD) and Multi-lingual (10+ languages). I have to be honest, I thought "Why isn't this guy booking in a hotel or Airbnb Plus?". But fine, maybe I'm pre-judging the guy, his other 7 host reviews seemed very good to me, anyway I have instant booking so I couldn't cancel if I wanted. In that same message he says he's allergic to smoke and asks: "I hope there are no smoking neighbours?". I sympathized with him and tried to assure no smoke smells were left from previous guests, although smoking is not allowed inside of the house, but we never know, right?
Fine, he arrived I showed him his room and gave him a tour in the house. I said I live upstairs and he could contact me if he needed anything. Few minutes later, he sends me a message "is there an electric kettle for boiling water?", I told him I didn't have one, showed him other options to boil water and told him I'd buy one in the next time, which I did (all stores where closed at that time). Next day, he sends another message "the water flow in the bathroom washbasin is very very Low. Is it possible to check and improve the water flow?". I apologized to him and mentioned that I had planed to do a bathroom renovation to improve that flow, but for the moment I couldn't do anything, it takes about 5 seconds to fill a 100ml cup. I had 100 other guests in this house and 3 other living at that very same moment and they were absolutely fine with the water pressure, I even got a 5 star review from one of them who stayed 3 days in another room at the same time as this guy (the other two are still there) and many others before. One day later I found out that he had closed the hot water valve below the washbasin, possibly trying to solve the issue himself, when other guests mentioned there was no hot water there. I opened it again and everything was back normal.
He seemed OK after that, asked for more stuff in the kitchen which I pointed him where to find. He used well the kitchen, stayed late hours watching TV with his tiny short in the living room (that was mentioned by other guests). Overall looked like he was enjoying his stay. In his last day he asked to check out few hours later than the allowed and I happily agreed as I didn't have any guest booked for his room in that day. I personally went to talk to him to check how was his trip, if he had enjoyed Montreal (which he did not) and socialize once again. Everything seemed fine. I even thought in giving him 5 stars.
Three days after he left, Catherine from Airbnb (Case Manager from Trip Department) contacted me saying the guest had opened a case about some amenity issues:
1. Very low water flow
2. Washing machine was broken - never mentioned by the guest. I have a note on top of the machine asking guests to contact me if they want to use it so I can help them to use my personal washer, many others already used and were glad.
I explained her the whole situation, showed her pictures, asked her to contact other guests and send investigators to my home to see for themselves all to take a fair decision. I also asked her what the guest was trying to achieve with this opened case? Did he want to close my listing until the water is by his desire? Did he ask for refund? She told me he did not ask for refund, but she explained him he would be eligible to get one, "The guest have the right to know the Guest Refund Policy as it is part of the terms and conditions we have with Airbnb just like the Hosting Standards".
https://www.airbnb.com/help/article/544/what-is-airbnbs-guest-refund-policy-for-homes
I asked which exactly hosting standard I was failing to accomplish, she said "this would fall under amenity (basic needs of the guest). Water should be available and operational." If it wasn't available and operational all other guests would have complained (specially the ones living there at the same time). Why the guest stayed the 5 days?
Anyway, she decided the guest should be refunded 30% of the nightly rate for the inconvenience and "the guest was more than wiling to resolve this issue" (of course). Airbnb is paying half of it and I'm paying the other half. I just can hope if he writes a review he writes a honest one.
I knew there were many guests out there asking refunds for many different reasons, but I was naive to think that would never happen to me.