This happened a while ago while I was traveling to the US on a 9h flight from Europe. Context: I'm Greek / Lebanese and am a rather moderately hairy person, most of it on my face.
I had an important meeting to attend in the US that was happening a few hours after I land, so I had to freshen up and prepare while on the plane still. A few hours into the flight, I went to the bathroom to change my clothes, wash my face and freshen up, trim and maybe my beard, put on some deodorant, etc. I took my backpack with me since I've got everything in there.
After spending some 5 minutes in there, struggling to move around the tiny bathroom stalls on planes, someone knocks on the door. I don’t answer, I thought someone needs to use the bathroom but it was clearly occupied. Another 5 minutes go by and I start hearing chatter outside and could feel some people moving around. I hear a knock again, at this point I had my shirt off and my face was half shaven with mousse all over the other half. I open the door in this questionable state. To my surprise I see 3 flight attendants looking terrified standing around the door... Two women and one man.
So I laugh and ask them if there are a rule and a timer for how long you can use the bathroom or what exactly was going on. The man proceeds to tell me some passengers complained because they saw me go in with my backpack, and I stayed in there a while, so they were scared something was going down and reported it to the flight crew. Mind you it wasn’t even at the "questionable" beard or stage or anything... it had grown for some two weeks or more give or take. So I laugh some more and tell him I fully understand, but I had an important meeting upon landing hence freshening up, and that I’ll need a bit longer to just shave fully and finish grooming myself. He then excuses himself and we laugh a bit then he goes away.
Shortly after he comes back knocking and I open, this time in a new fresh shirt and fully shaved, and I ask what’s up? He tells me that some passengers are still concerned I’m shaving my beard and thought I was shaving my body too since I opened the door with mousse dripping on my face and without my shirt on.
I was very confused and at that stage started to get annoyed, just let me use the damn bathroom in peace there are many other people can use. Turns out that apparently some extremist Muslim groups do this before they "get up to no good" (aka sacrifice their lives in an act of holy retribution/terror/whatever the fuck you want to call it - paraphrasing what they told me). Something about going to heaven freshly clean... So at this point, I’m laughing too hard but I tell him I fully understand and that this is a good thing they check on such instances. I’d rather be safe than sorry. I then show him my meeting email with the time and date for the sake of their peace of mind. I also mentioned I was Christian born but not religious and that even as a Lebanese dude I had no idea terrorists were shaving before acts of terror, thanks for the information I didn't really need.
We then had a chat outside with the rest of the flight crew, laughing and making jokes. I could tell they were still a tiny bit paranoid but 95% apologetic. They didn't check my bag or anything and now that I think about it I should have shown them what was in the bag (snacks, iPhone cables, some books, clothes, and my toiletry bag).
TL;DR - I groomed myself on a flight and the flight crew along with some passengers went full anti-terror alert mode while fearing this was something more than just one beardy dude shaving and putting on fresh clothes.
For the many "why would you shave on the plane" queries - it was a special circumstance trip that I booked on a day's notice and my beard grows too fast so shaving before we took off would mean I'd have some annoying stubble when we landed. I wanted to look as fresh as I could for that meeting and I did what I could with the given circumstances. I also groomed myself during a “lights off” time where everyone is asleep. Queues to bathrooms don’t form during that time. Remember, they offer you shaving kits on some business flights so to think anyone shaving on a plane is extreme is funny but understandable if you’re unaware. They have plugs for razors, hairdryers, etc in the plane bathroom stall. I mean they change poopy kids in there so think about that for a second Reddit.
I can't believe this story blew up.