In most cases for most hotels, simply stowing your important items within your bag is fine. But for whatever reason, this may not be good enough.
Personal locking safes within rooms are far from actually safe, it’s only there to offer the guest a sense of (false) security. So, if it’s absolutely imperative that you need to keep certain items away from everyone, you must not keep it in the room in the first place.
The hotel staff, particularly the housekeeping, know every centimeter and crevice of the rooms they so diligently keep. It must be assumed that any hiding spot you may cleverly use is already obvious to the staff. Unless you can proficiently dismantle structures or appliances to keep items in them and then reassemble to the original condition; televisions, fixtures, furniture, frameworks etc.
However, it must also be assumed that an operative, agent or other trained professional will find what they want to find in any given hotel room.
The key to keeping items truly secure in your hotel room is by not having it in the room. The special items should be secured close by in an unaffiliated (to you) external area but positioned in an obscure space:
– Under a large trash bin of the hallway of your floor
– Hanging by cord or magnetized on the side of your room balcony
– Magnetized under the elevator car, accessible by stopping the elevator short
– Under a hallway couch or seat with a low profile, check for dust
– In an alternate hotel room registered under a different name
– Magnetized under a vending / ice machine
– Hanging off or on the window sill of a low traffic staircase, check for dust