First Poster here-

LONG READ

So I live together in a big house with five other people (so 6 people in total). Each of us has his/her separate bedroom and we have an extra spare bedroom if friends come over.   
So this has been going on for great for year and a half now. This week however my roommates were on ski-holiday and celebrating new years eve in France together. I couldn't go because had to work the 31st Dec and the 2nd January but no hard feelings and i wished them the best.  


So this new year eve i get home with my girlfriend at 3 am because we celebrated with her friends and I see that the front door is open which was weird because i always lock it extra when I leave. I rushed in the house to see if anything was missing but everything was in place. Already then I thought something was off and I was mad at myself because I thought I didn't close the door well and you things could've gone wrong. but again nothing was missing. So we go to sleep because it was already late and we got woke up at 5 am because I hear rumble in the hallway. I got up because in my sleep I thought it were my roommates that because of some reason came home early from their ski-trip (which was weird because it was new year eve). So I open my bedroom door and I see 3 guys I have never seen before in my life  going up the stairs. I thought they were burglars  so I went after them and yelled at them that they better get the hell out of my house, which to me is a normal reaction when 3 strangers are in you house in the middle of the night.   
One guy turns around and says" I'm X, I'm the brother of Roommate. Didn't he tell you we were staying here tonight"  
I already heard about X before but never seen him before and sure as hell didn't know that he and his friends were staying over tonight.   
X has a weird name in real life so I knew he couldn't have made it up so this was really the roommates' brother. I'm like no WTF i didn't know? Why didn't roommate tell me?  


So i go to my room and calm down my girlfriend because she got her phone out to call the cops ( which is also a normal reaction and is in hindsight even a better reaction than mine)  
I call my roommate in the middle of night because I'm pissed off and why he didn't tell me because it never has been an issue to have sleepovers before. He's like sleepdrunk and is just going I'm sorry i forgot to tell you. So next morning I hardly slept after it because I've been so scared that night and so full of adrenaline. So i get up,still upset about it all start doing regular wake-up stuff breakfast,coffee,etc So at noon the 3 guys were still sleeping and i woke them up because I wanted them to go home because I've had it with the whole situation so I told them: Look guys it's better if you start packing and go home because you scared me to death last night, it's not your fault  but I'm really don't want any visitors in my house at the moment." So guys get up, grab their stuff and leave. It was then that i noticed that they had so much stuff with like air-mattresses etc so Then it came to me that they must've come in last night (because roommate gave them a key) when I was already gone and left the front door open.   


So last night roommate comes back home from ski-trip and wants to talk to me about the whole situation. So basically he tells me that I've shouldn't have reacted that way to strangers in the house in the night. He says I've just should've gone up to them and asked who they were and what they were doing here.  
He says I shouldn't have called in the night either because he felt bad the rest of the day and that I shouldn't have told his brother to go home the next morning because he's an insecure snowflake and got scared.  
So I tell him that I don't have any issue with his brother at all and his brother is welcome but that next time i would like to know beforehand (like we always do). I'm not telling him that he has to ask permission to let his brother sleep here I'm just telling him that I would like to know next time so I don't think we have burglars in our house.  
Then he says he didn't think I would be sleeping home that night so he didn't want to bother me with it which to me is also wrong because I think I have the right to know when someone is in our house. It has never been an issue before to let someone sleep here. The guys even would have gotten eggs and bacon the next morning if I wouldn't have been this upset.

He doesn't seem to get that I was really frightened that night and reacted out of impulse (fight or flight) and that the whole situation came out of him not telling that his brother would be sleeping over

TLDR: roommate let's people sleepover without me knowing and it frightened me because i thought I had burglars in the house and now he is mad because of my reaction