Can a cat be FIP+ with no symptoms?

Last week our 7-yr.-old calico was lethargic and constipated (zero poops) for 3 days. Vet did scans and labs & said she had a high temperature, but was alarmed at her albumin:globulin ratio which she described as "way off" and a red flag for FIP. One enema and lots of poop later, the cat is 100% back in her old form: big appetite, regular BMs, energetic & playful & goofy like before. Today, we went back for follow-up labs to recheck the FIP possiblity. Will know in a couple of days.

Is FIP a common misdiagnosis? And can this cat have it with no noteable symptoms?