I have one of these Key Safes Online - Australian Key Storage Solutions – tagged "Category_Wall Mounted Key Safe" (second one down)
How about attaching a decorative sign to your front door? A plaque, or a wood carving with “welcome” in a foreign language, or with your name, or just the house number.
Then nail the key behind the plaque, and hang the sign with glue or 2 very small nails that can be ripped out.
(Or if you happen to be Jewish, you can nail a
[ritual mezuzah](mezuzah - Google Search)on the doorpost, and hide the key inside it) 
You don’t have a single friend where you live? Not one? No reasonably friendly coworkers? Nothing? Dude, that’s a security risk of it’s own.
If this helps at all, I simply don’t let my doors auto-lock. Like the doorknob locks that can be set in the locked position so when the door closes behind me it’s already locked. I don’t do that. I make myself lock the door with the key when I leave, that way I can’t possibly lock myself out. It’s worked well as a policy for 20+ years so far.
Failing that, I’d probably go with one of those Realtor’s locks.
I’ve made key hiders, using a waterproof, airtight bottle and gluing a stone to the top, then it is buried in a flowerpot with only the stone showing. Depending on the size of your key, the bottles from diabetic test strips work well.
We have a garage with a key code, and the spare key is well hidden in the garage, so there’s another layer of protection there.
in your underwear
For Og’s sake really???
He is going to misplace a hidden key? Are you kidding? And some one is always watching? Seriously? Just choose a quiet time and be discreet.
Hiding a key is not poor practice. Where do you get this? Most locations have oodles of spots to hide something small like a key and it increases tenfold with every square foot of property available. Burglars do not go around looking for hidden keys, they break windows or force doors. Its easier.
This sounds like an apartment. Is there no place on the outside of the building with a concealed surface? A downspout? Underside of a lower fence rail? A nook in the branches of a shrub? Switching out the deadbolt for a keypad bolt as others have suggested is pretty simple. You can have it keyed the same to keep the landlord happy and take it with you when you move on if you like.
Swindon looks like a nice little city.
I have something similar to this. Its great - it knows when the deadbolt is engaged or not and allowsyou to lock the bbuilt from the outside without the key as well. If installing a key fob were cheaper of do that so I never have to reach into my pocket to get my keys. You can also program guest codes for convenience that can be deleted when desired.
Put a spare key in your wallet. You may walk out of the house without your keyring but not your wallet as well.
But if you do, make sure you know which is the window to break to get in.
EDIT: Oh, location specific. The only thing that is coming to mind is find a creative way to attach the key to a non-obvious part of a trash can or recycling bin that you know will be there. That may be too much as it may seem conspicuous to an unwanted neighbor to see you retrieving it.
(May have been covered already but…) DON’T hide the key under the mat, under an obvious looking false rock, under a potted plant, or above the door post.
If there is a park or public building with some sort of yard or carpark nearby poke a brass key in the ground at the base of a memorable post/seat/tree. If anyone ever finds it they won’t trace it to your door.
If you have a locked mailbox tape it to the top of that and hide a duplicate mailbox key instead for an extra layer of security.
Back before cars had smart keys, I always kept a spare car key taped to the inside of one of my hubcaps.
I missed the editing window: experienced criminals would likely think to look in these places.
really, go ahead and try it. use an old door key that doesn’t really open anything. put it somewhere public and in plain sight but not a place where a cleaner might find and throw it. it’s going to remain there for a long, long time.
A garage was my first thought, but it doesn’t appear that the OP has one.
My spare house key is in the garage. I have a stack of seedling cups hidden away behind some other stuff. My key is in one of the cups in the middle of that stack.
Not sure if it’s already been mentioned but my old neighbour used to hide his spare key along the top of his screen door. There’s a little crevasse that a key fits perfectly in and nobody knows it’s there.
For a high-tech solution: get a door lock that opens when a mini-RFID chip is close by. Embed that mini-RFID chip under your skin in an appropriate place (under the skin in your upper arm, for example).
With this solution, you will never even need your keys to get into your house.
J.
Tape it to something that is outside of your house, like the air conditioning unit or cable box. Use black electrician’s tape and wrap it several times to completely hide the key. if you have a car lift the hood and tape it to a wire or cable.
Make sure to fold over a ‘tab’ so you can easily unwrap the tape if/when you need it.