If you get mugged you've lost your passport and money and keys. The passport and money can be dealt with in the usual way (whatever that is -- sobbing in the street?) but you're going to be sat there facing a bike which you can't move.

Sure, you could ram a screwdriver into the ignition but that's just adding insult to injury.

What you want to be doing is stashing a spare set of keys on the bike. This gets a bit interesting. One obvious place would be under the seat as that is secure, locked by the key...oh, wait.

You need a better place, secure-ish, ie. unobserved and yet accessible without the key.

Some people have suggested under some resin or goo (for waterproofing) behind the number plate but I tend to think the the number plate is something relatively vulnerable on a bike and in the event of it breaking off I might not be compis mentis enough under the circumstances to remember to go looking for my ripped off number plate to retrieve my spare key.

On my bike I found a spot at the front which isn't visible (so I can't even tell if it's dropped off!) and should be fairly waterproof under some gaffer tape. To gain access I require an allen key which should be fairly easy to get hold of.

My other spare keys are then under the seat.