You need to bear in mind that everyone who comes to the door is not always genuine. Bogus callers come in many guises; you can´t tell by looking at someone whether they are genuine. These are some handy tips, courtesy of a locksmith we use, Hackett Locksmiths, 966734517, that should be followed when someone unexpected opens the door.
Opening the door
If someone rings the doorbell, the door or gate should be kept locked until they are identified. If the caller is not recognized, they should be asked to identify themselves before the door is opened. If you are in a villa with a locked gate, make sure you close the front door behind you but remember to take the key!
Caller should never be let in from companies until their identification has been checked and verified.
A bogus caller may try to distract a householder out of their home, for example, they have noticed something wrong with their property or have a problem with their keys. This could be a ploy for their colleague to get into the property without being noticed.
Don´t be bullied by thinking you have to let someone into your home, especially if they say they are from the Town Hall or another company. I recently had a gas company come round. They said they were an approved company by Repsol to check gas appliances, which had to be done legally. I asked for ID and they showed me a business card, but the name was suspicious and it had no website, just a strange email address. I refused to let them in, and they went away (walking!), and when I phoned Repsol, they said they were an unregistered company (possibly not even a gas company) and to be very careful. If you are using bottles, you do not have to have the gas checked. If you have gas as an energy supply, the company will do it.