I'm sorry to hear Halloumi is missing!
You still can. If you have the microchip number in his vet records or adoption paperwork then you can register that with a company. That way, if he's picked up and taken to a vet or shelter they will scan him and your info will come up. If not, they'll scan him and either the rescue's info will come up, or it'll be blank (depending on if the rescue registered it with them or not).
Thank you. I spoke to the vet surgery at Pets at Home (where Halloumi was in the Pets for Adoption) and am going in to see the staff on the shop floor on Monday - I am working tomorrow - to check their records for him. Hopefully they have kept some.
I would ask all your neighbours, ask at all your local vets - leave a poster with them, put up posters on lamp posts, ask in local shops if you can put up notices, ask at any local animal sanctuarys, even if they don’t take rabbits normally. Facebook is a good idea for spreading the word too, although a lot of people don’t use it. Your local paper might let you put a lost ad in.