If at some body your doorbell is ringing without you expecting any guests it might be someone from the gas or water company to check the gas meter. Upon checking the meter you’ll either get an invoice right away or find it posted a few days later on your front door. Take this red or white paper slip and go to any bank or post office to pay it within max four weeks. If you’re not at home by the time the company visits, you will find a notification on your door which asks you to check the meter yourself and afterwards call them to tell them the meter count.
For general security reasons please make sure you know where the main gas valve in your apartment is and how to turn it off. If the landlord hasn’t shown it to you yet, a visit of the gasman is a perfect opportunity to find out.