It is possible that the foster has removed the Rescue account from their Fi app, taking away the Rescue’s visibility on the dog. Similarly, if they set the collar up themselves, if they do not add the Rescue account as an owner, the Rescue will not be able to see the dog.
If you cannot see a particular dog, please reach out to rescuesupport@tryfi.com with the missing foster’s collar serial number so we can look into it for you.