I know I am a smartass but there is a method in my madness. I have a plastic clip thing on my bike key which I use to attach the key to either:
a. a ring on the sleeve of my jacket when wandering around off the bike but in biking mode
b. the buckle of my trouser belt when not wearing a jacket but still in biking mode
c. the rest of the bunch of keys for the bike when in the key safe/tossed on the sideboard when not needed
If it isn't in the ignition, a pannier lock, the seat lock or one of the above, then it is officially lost but the discipline means it almost never ends up anywhere else. Except in my trouser pocket in the wash on odd occasions...
I also have a cute(ish) braided coloured string thing on the key ring which I found in Waitrose car park decades ago. Some little human had spent ages weaving/knitting/knotting/sewing the Scooby Doo (Scoobidou? -
https://en.wikipedia.org/wiki/Scoubidou ) like thing and then obviously lost it. I thought it too nice to bin and so put it to good use as an ornament. It is surprising how often I see the stringy thingy poking out of somewhere odd and know where my key has wandered off to when the method fails me.