The UV lights only work on dried urine - so you should be able to use that to ID the source. Depending on how bad the issue was, it could have soaked into the plaster and/or insulation (if present) so you may well have to tear at least a bit more out to remediate.

Animal urine can be a real challenge. I had a tenant in one of my rentals who kept a dog without having it in the lease. Apparently she didn't bother to let it out at least part of the time. After she finally vacated (behind on the rent), in addition to the usual floor cleaning/repair/sealing, I had to have the wall between the kitchen and living room completely demolished and rebuilt, studs and all.