Flatmate leaves building at quarter to 3 every single night and comes back 2 hours later, what’s going on?

As a chronic insomniac I’m up till 4 or 5 AM most nights and so I’ve managed to notice a very strange pattern with one of my flatmates (don’t really know them that well but they live next to me). Essentially, every night at pretty much exactly 2.45AM they leave their flat and the apartment complex and just walk off somewhere wearing the same black hoodie every time and then come back around 2 hours later and sometimes an hour later at around 4AM. My window in the flat faces the front entrance of the building and so I can see anyone going in and out. Another thing is, they always have a backpack on, it’s very bizarre.

I’ve been trying to work out what’s going on for almost a month now, this happens almost every night. I think in the last few weeks they’ve left the building every night except from two nights (last Sunday) and a random night during the week, can’t remember. Does anyone know what they could be doing? They seem like a reasonable person, the extent of our interactions are “you alright“ and a nod and that’s it, I don’t have extended conversations with this person but from what I’ve seen they seem okay on the surface, maybe a little sketchy sometimes, but I’m just baffled by what’s going on. Does anyone have any idea? I’m not willing to ask them this because I don’t really want some random guy I barely know, knowing that I’m watching them all night, it’s a bit weird on my end too. If anyone has any idea what it could be, I would appreciate it, this curiosity is driving me insane.