Just a cautionary... while forum MYTH often suggested disconnecting the battery to solve some mystery electronic Gremlins this really is not the best idea and, in most cases, because the ECU and other onboard computers hold information in memory disconnecting the power supply will serve no real purpose but if not carefully done could cause additional issues.

sorry schmieg I read your very helpful responses but this one made me go ugh... lol

No problem. I've used this several times on the Jeep and the MX5 with great success, but it depends on what the problem is. I've never had any problem with it, but I always carefully remove the negative cable and isolate it from contacting metal surfaces. I usually also press the brake pedal for about ten seconds and then wait ten minutes to reconnect. This has solved a number of electrical glitches, especially in the radios or memory seats not working properly (after doing the memory seat reset to no avail on the Jeep anyway).

The main problem with it is that the engine will run rough for a while until the car relearns things, but that doesn't last long and usually results in a smoother running vehicle after relearning. The Jeep will retain radio memory, but all the other stuff that is controlled through the radio gets reset and that includes the sensors for blind spot, parking, memory seats, key fobs, puddle lights and a plethora of other crap that isn't really necessary to the vehicle, but which can be irritating when it isn't working properly. The MX5 is a simpler vehicle and my CX5 is a 2014 GT which doesn't have all the bells and whistles now present in the newer ones (thank the gods).

Mazda
 

Have you figured out the beeping?
my 2016 CX-9 just started doing this and it happens anytime I get in or get out of my car. Any suggestions? Thanks!

I just had mine do that the other day, but it was because I left the engine running and exited. Three beeps. However, that doesn't seem to be the case with the OP unless the computer somehow thinks the engine is still running.

I'll check that out tomorrow. My car is definitely off when it beeps. I can exit immediately and it'll beep or if I sit there in the parking lot or garage on my phone for a few minutes after shutting the car off, it will beep when I exit the vehicle.

The three beeps are usually a warning that you have left something on that isn't going to shut-off automatically.
Exiting the vehicle with the engine running will do this, happens to me when I start it for below freezing temps, and go back in the house to stay warm.
It will beep if there is another key FOB still in it. Though normally that only happens if you lock the vehicle.
There are also a host of other potential reasons for beeps for a not running vehicle from minor things like bad FOB battery, doors not closed, not in park to actual electronic issues.
And for a running vehicle there are a huge assortment of reasons for beeps.

I've experienced that on my Jeep for quite some time. I usually lock it with the button on the inside of the door and then get out and close the door. If I do it too fast, the Jeep unlocks and beeps three times. I agree with your analysis of what is happening.