Can workmen/repairmen after completing their work (saying goodbye and done for the day) come back in the apartment? Can they enter and walk into a bed room where I was undressing?
The repairmen can't get into your apartment after they completed their work. That's more likely to be a break-in since it's definitely not okay. First of all, you need to call the police and tell them about the whole situation. In general, the person above wrote everything pretty detailed, and I think you should do everything they wrote about. Also, I'd recommend you install some kind of security system. For example, I have https://ajax.systems/ in my office, and you can try it for your home.