How did you turn data off?  It is possible that it was a delay in recording data use. Did you track on your phone or in self-serve? 

Settings > Network & Internet > SIMs > Mobile data

I turn on data for a few minutes to check email, for example, then turn it off. But I continue to get email and other notifications, and self-serve shows data being used, as does my phone.

On Saturday I turned on data at around 2 in the afternoon for a few minutes, and got a Koodo notification that I have used 50% of my data. The next day, without data have been on for at least 18 hours, I got a notification from Koodo that 90% of my data had been used. I was in a remote area with spotty cell service, so I suppose it is possible that this was a delay. But there is no way I used more than a 100MB of data in those couple of minutes it was on the day before.

You’d be surprised what a smartphone with updates can burn through, unless you set it all to update only on wifi and/or by prompt. 

So if you go back right now, and check your data, is Mobile Data still toggled off? Was there any prompt to confirm? 

Do you use multiple sims?  Could you be turning data off on the wrong sim line?

Doesn’t explain why when I turn off my data, the phone continues to act as if data is on. It does not show that it is connected (the mobile data is toggled off), but email headings and other notifications continue to come in. It is not clear how long this phantom data-on state lasts; eventually it does really turn off.

I only have one sim card.

Regarding unexpected data use: when I turn on my mobile data it is almost always to check email, which does not use very much data. I have no social media apps, streaming apps or other network intensive apps like google maps. All updates are set for wifi only. I have used my phone this way for years. I have tracked my monthly mobile data use since 2014, and I normally use about 5 MB a month on average. I have had a plan with 250MB free data since August 2019, and have never once come close to using that. What I am observing is something that just started happening over the last few months. This warning I got the other day about hitting 90% makes no sense. I have never seen that before, and have not been using my phone any differently.

Did you check in data usage which apps used this data? It does go into more detailed tracking by process or app. 

It only shows usage over the entire usage period. I am coming to the end of my usage period so while the graph showed a huge spike a couple days ago, I cannot see which apps used how much data on a particular date.

However I did notice that a large data hog (Outlook) had mobile background and unlimited mobile data turned on. That should not have been set that way so I changed it. That said, still does not explain the phantom data issue (data staying on after I turn it off)