Spare key.

I have had a key safe for my front doors for many years. My house in East London, had this fitted into the porch. Absolute blessing and used (by me) on so many occasions.
One winter night, I took out the rubbish to bin. Meant to leave the front door ajar and close the porch door to keep in the heating, but did it the other way round! There was me, in nightie and dressing gown standing in front of house unable to get back in. Moments of sheer panic!!!! Could not even remember the phone no of the daughter who lived about a mile away, so no point of going to a neighbour.
Then I remembered the key safe, and in a couple of moments was safely back in the warmth.

ANother time was back in 2012 during the para olympics, I was many miles away visiting an AC, My Adult twins had tickets but then found out that th e friend they had intended to stay with was ill. A quick phone call to me, and given the number of the key safe, meant they were able to use my house.

In the flats where I now live, I still have one by my front door. Do sometimes, go out forgetting keys, no other way into flat. It has been used on several occasions/

a key safe is the way to go. It has a combination lock system.. you choose 4 digits and roll them into position a bit like a fruit machine! give the number to nhs/ health workers to keep on file should they need to get in. of course it's very useful for yourself if you go out without a key.

Yes Ive seen fake rocks in which you can hide a key - suitably wrapped in polythene. If you have a garden with lots of rocks then its is unlikely to be found in a pile with others or garden debris. Or hiding the key in a recepticle of some kind taped to bushes or shrubs. You would need to know where to look.

I had a break in some years ago when I lived on a rough council estate. I was going in holiday and had cash notes hidden in the leaves of one book and my credit cards in the other. However I had several shelves full of books so they were hidden in plain sight. The thieves took a ghetto blaster type radio and a couple of pounds I had in a purse. That was all they got.

Criminals are usually in a hurry and not renowned for their intelligence.