I am a very troubled boy, and I hope that you can help me.
There is few things more upsetting to a cat than moving house. There you are settled and happy in your territory, when suddenly your pet human stuffs you in a cat box and turns you out into a strange new place. All the smells are wrong. You don't know where anything is. And it feels very very unsafe.
Most of us hide under the bed for a day or two. But some sensitive cats like you suffer from stress-induced illness. All that going to the litter tray is, I fear, a sign of cystitis. Vets (loathsome people) call it FIC, Feline Idiopathic Cystitis. "Idiopathic" just means "we don't know the cause." But we cats do know the cause. It's stress due to house moving.
The earlier trauma of getting lost will not have helped. That must have been unsettling and probably also very frightening. Then this... poor Thomas. Even loathsome vets know that moving house is stressful for cats and can result in FIC.
Having to share the corridor with another strange cat (who hisses because he is frightened too) is another stress. Perhaps your humans could invest in a cat ladder letting you go outside from the window. Or put several cardboard boxes with entrances in them, where you can hide if the strange cat passes by. Or just keep you safely in the flat till you have recovered. My human's website has some suggestions for how to make indoor life more interesting for you.
Why humans insist on changing their territory is beyond my imagination. They are a very insensitive species. Your letter makes me feel angry about their ridiculous behaviour.
They must reduce your stress and read the instructions at http://www.catexpert.co.uk/cats/reasons-why-your-cat-feels-stressed-and-unhappy-and-what-to-do-about-them/
Yours with sympathy,
PS. We hate vets but this is one time when you need their help.