530 keeps auto starting whilst inside

I'd add a speed sensor.

You are correct in your thinking.  Just turn the unit off when you are inside.  Why would you need it on when you aren't riding a bike?

If you must have it on inside use the trainer profile as that turns the GPS off, or turn the GPS off in whatever profile you are using.

I am charging it on my computer, once I unplug it, it switches on and then starts a workout. I also switch it on inside to change settings, update it etc. I am not a complete idiot.

There is a stupid pathetic design fault with a Garmin 530 whereby it does turn itself on after the power supply is turned off or its unplugged from it. Many a time in the past I have got up in the morning to go for a ride or a day or two later and find the battery is flat or almost flat because I did not turn it off from disconnecting from a power source What other electronic device does this, non that I know off, just another Garmin "Thinks its a good idea" but serves no purpose whatsoever excepts messes people about.

But, starting a workout is unusual, unless you have one running and it needs stopping completely as otherwise the workout might be just resuming from when the 530 was turned off.

I myself do the same and run it in the house to alter settings, check course etc in preparation for a ride the next day. And oddly enough, a few nights ago for the first time whilst checking a course, mine switched off and did a restart for no reason at all.

I am guessing at the loss of gps signal and regaining it being the the cause of the auto start. Kind of makes sense. It would be interesting to know if Garmin know about this  

When I unplug my 830 from my computer I make sure I turn it off.  Unplugging it from a computer won't start a workout.

If you plug a unit that is turned off into a wall charger it won't turn on and will be off when you unplug it.  It is plugging them into a computer that turns them on when you unplug it.  I agree it is annoying they do that.

I’m not too sure you are understanding/ listening to the problem I have. 

I do understand.  These units turn on when you unplug them from a computer.  It is annoying but they won't start an activity when you unplug them from a computer.  That is completely down to weak GPS signals making the unit think it is moving and triggering the autostart function that you have on.  If you do need your unit on inside because you are changing settings go somewhere with no GPS signal at all or turn the GPS off in the settings to stop the weak signals triggering your auto start.