I will call the landlord no more!
(Make sure it's weird enough. A block down the street underneath someone's front steps, under a mailbox that is not yours, something like that.)
Firstly: Leaving your key in someone else's property is a good way to lose it.
Secondly: Burglars are very aware of people leaving extra keys around, and what you may think is a super hiding place is actually #4 on the burglar's Typical Clever Hiding Place list.
Duct-tape a set of lockpicks somewhere weird instead, if you're going with that. No biggie if you lose them, and a burglar is either not going to have the skill to use them, or carry his own, better set instead.
Yup, quite possibly. The return/effort ratio for doing that is pretty good. The thief doesn't need to get in the same day as s/he finds the key, after all.
I once saw a shady character try the same key in every car in a parking lot.
You can take advantage of logistics as well; for example, hide the key somewhere in/on your car. A potential thief would need to steal your key in advance, lest the key simply be gone when ever you're gone.
Another ruse: a quarter superglued to the sidewalk in front of a local university's library.
Leaving a door key under a potted plant is indeed a bad idea.