Why do some hotels ask you to leave the key at the reception when you go out to come back later?
In the past (1970's/80's), for smaller hotels and pensions:
- prevention of loss (they were rather ugly and heavy for a reason)
- prevention of strangers 'wandering' around, where the reception is strategically placed near the main/only entry
- telefon calls: with the then existing older systems where a manual connection was required, one simply told the caller that the guest was away if the key was there.
Hotels then often had a 'long memory', so people working there were often trained on what to look out for.
Maids would report to their Matron/Housekeeper and they would report to the receptionist anything that looked odd (the main source of the hotel grapevine that was very fast and often more extensive than strictly necessary).
Often, someone in the hotel knew how to exchange locks (and other general repairs), which would be done if a key was lost under suspicious circumstances. Thus a stolen key would no longer match the original room and become useless for a hotel thief. In the small hotel I worked at during my studies, the owner did this himself keeping track of all the exchanges. There were also spare keys/locks, so it could be quite some time until a stolen key would be used for another room.
So there were good reasons for this policy, which served not only the interest of the hotel but also of the guest (in public statements: the other way around).