Did he scratch like this before he was missing for a day? Have you bathed him since all this began? I'm wondering if it's not the collar but something else going on with him scratching. It's been very warm recently and it is allergy season. My suggestions would be bathe him in an anti-itch shampoo and if it still continues a vet visit. I am a very strong proponent of ALWAYS WEARING A COLLAR. There are just too many stories of dogs getting out and never being returned and it always seems like they were not wearing their collar with ID tags.
Chipping is not enough~IMO. People need something to see immediately if they find a dog.