As someone who runs a rescue and chips every single cat in our care and then registers it, I have been on the end of a shelter contacting me for someone giving up one of our cats to the local shelter vs returning it to us as it states in the contract. In my 30 plus years in vet med, I have seen too many cases of stealing pets even with chips and having to get law enforcement involved, ER's and GPs' not wanting to check because they don't want to be on the receiving end of dealing with who is the pet's owner etc..  and not wanting to trace a non-owner registered chip- but that chip can be traced to placement simply due to the "headache" it can cause. 

If we are recommending permanent ID, we should be checking new patients each time that come in for care, as well as patients that are DOA from road side accidents etc... our scope of care DOES NOT STOP because the person presenting the animal is not the owner or has to pay for services in order to get any type of care, including a chip check. 

If you aren't already checking for a chip, you are providing a sub-level performance of medicine.