When you go to see someone at Harmondsworth Detention Centre near Heathrow, you sit in a waiting room until your ticket is called and you are led into the large visiting room. After a while, the person you are there to visit enters through a door on the other side. The waiting room and visiting room are decorated with photographs printed onto canvas. The photos are the stock kind you might get on the desktop of a Windows PC: deserts, beaches, lush forests, drops of water, lands of mineral richness. They appeared after Mitie took over running the centre from GEO (both are private companies). Artwork by detainees used to decorate the walls, but now those pictures are stacked up in the corner.

I don’t know which policy I find more disturbing. I suppose it’s nice that GEO allowed the detainees to decorate the walls, but they shouldn’t be here in the first place. Harmondsworth is the largest immigration detention centre in Europe, and Britain is the only country in Europe that has no time limit on the detention of asylum seekers. In June the High Court ruled that the Fast Track system was ‘structurally unfair’. Under this regime, detainees, many with poor English, were given only a few days to navigate a system of deadlines and applications before being bundled onto a plane, or being left to languish in detention. But even without Fast Track, detention is a confusing, complicated experience.

There’s something sinister about detainees decorating the site of their detention, but then the visiting room isn’t really the detention centre anyway, only the public part of it, and pot plants and pictures serve an important PR purpose. The person you’re visiting may be detained indefinitely but the surroundings say: ‘it can’t be that bad.’ Perhaps the stock photos are better than the artwork. They convey no sense of the photographers who took them; they seem to have sprung out of nowhere. They can install themselves uncontroversially anywhere, even the walls of a detention centre, though the places they depict, which are meant to look decorative or exotic, may be the places from which many of the detainees have fled.

You can take pictures off the wall, but murals are more difficult to get rid of. One behind the entrance desk is an underwater scene packed full of colourful fish and coral. The guard, watching out for anyone passing things to detainees, sits below the gaping mouth of a shark – dark, circular and ringed by sharp little teeth. Next to the shark is a clock, hung in the centre of the mural and threatening in its own way. Some people have been in detention for years.

The person I’m here to see arrives. He keeps coming back to the idea that the experience of being detained is impossible for people outside, people like me, to understand. ‘I know it seems nice from out here, with all this stuff they put around,’ he says, gesturing to the pot plants and the murals and the pictures. ‘But it’s not like that inside.’