You can get external.lock boxes or key stores (normally used for old people or holiday lets) which mount to a structural wall and enable you to keep keys in such as https://www.amazon.co.uk/dp/B0091CYMDE/ref=cm_sw_r_cp_apa_glt_fabc_DQVS5V047VFBZQH8XSC8?_encoding=UTF8&psc=1
I can recommend one of these:
https://www.amazon.co.uk/Supra-Secure-Mounted-Outdoor-Storage/dp/B0091CYMDE
There are cheaper ones but they are all shit and open easier than a satsuma