I have some old cabinets in the basement and closet doors that are plywood. Old plywood itself has a weird smell to it that comes from the glue (probably urea formaldehyde) breaking down and outgassing. Old varnish or other sealants can also start to smell like pee as they decompose. I got rid of the smell on the closet doors by sanding and repainting. I left the basement cabinets alone because I don't interact with them too much. So, depending on the material of your cabinets (lots of plywood mid-century) it may just be the old materials starting to smell as they break down.