Could a trace of leukocyte esterase signal a uti ?

Yes: If this is the only marker that was present as positive on a urinary dip stick it does not have to be a urinary tract infection. A urinalysis with microscopy would verify if the leukocyte esterase is from an elevated white blood cell count in the urine or a lab error. If you have symptoms of a urinary tract infection send off a urine for culture and treat empirically.