Magnetic tankbag safety
 

I use a magnetic tankbag on my BMW but it worries me to leave it on the bike when I have to pay at a gasstation or go to a restaurant. It is to easy to just grab it and run away.
To make it a little more effort to steal it, I locked it with a small padlock on my handlebar. I covered the handlebar crossbar with black tape to protect it from scratches from the lock.
The padlock is right it the center of the handlebars so it does not disturb the handlebar movement at all.

That's a good idea. Of course a thief could still unzip it and have the contents, unless you lock the tabs together with another padlock.

I tend to leave the bag on the bike and make sure I sit where I can see it. You're right - it's a damn nuisance taking it off every time you stop. I think I will copy this idea. If you don't have a suitable crossbar, you could use one of those cheap cable locks for bicycles. I carry a couple to hang the helmets off the seat grab rails when we stop, and it works well.

Here in USA you can go to a cop shop (police station) and get a free gun cable lock. I have a couple of them and use them for helmet and whatever else. They are short, about 10-12" (25-30cm) long. They are metal, but that can be fixed with a simple rubber tube to prevent scratches, etc. I spoze in a pinch, you can actually use one for a gun...
YMMV

Just make sure you dont demagnetize your creditcards. Well its not so important after the chip came, but then again. I had to travel three days with no cards and only thirty dollars for gas in my pocket..

That's a good idea not only as an anti theft device but a fail safe too. Just in case it does come off.
Iv had one come off my tank on the autobahn and wrap it's self around my helmet.
It was a scary moment.:taz:

I can only second to the answer about the threat of demagnetising your debit cards, I've had the sad experience of carying a bag of magnets from an old audio device which was placed close to a pocket where I kept my debit card. Luckily not on a journey, I actually was not able to withdraw any money from an ATM, drawing the conclusion that it must have been due to the magnets in my bag. Otherwise, this seems like a solid nice idea.

add a strap to tank bags
 

every magnetic bag, case, etc i have used has been a hazard in high winds or even just a strong crosswind/gust at highway speeds. they all can blow off or flip up in the right wind conditions, making you grab and hold them down or go back to pick them up. straps on a bag are a pain, but i have never had problems with losing them. outdoor shops sell raw strapping and the quick release click-snaps to add to mag bags. straps don't have to be super strong, just enough to hold in the wind. i usually just un snap and take the bag in with me to eat, since it has my maps, electronics, etc. and also have a place to stash any leftovers for later

I use a very small magnetic tank bag in which I keep all my important papers, tickets, passport, money, credit cards and camera. Cards are all the chip type. The reason that it's magnetic is so that every time I go more than 2 metres from the bike I can take it with me. It takes 2 seconds to get it off the bike but the magnets are very strong and I don't believe there is any risk of it becoming detached by wind.

Another Option
 

My map case is a Walmart plastic case for that very purpose. A couple of heavy duty magnets. Voila!
I use a fanny pack tied around the mirrors. Small, compact, cheap, easy to replace and a quick release. Holds my sunscreen, glasses, spare plugs, and toilet paper.
Or go with cargo pants. It is amazing what you can fit in those pockets.