If a doctor finds “trace blood in urine” but the female patient has large hemorrhoids that bleed, could that be the cause of the microscopic blood?

Possibly. The only way to determine whether the blood comes from the rectum or not is to try to get a clean catch sample of urine that prevents contamination from anorectal or vaginal discharges. Wash off the vagina and place a cup against the urethra to collect the urine specimen. A urinalysis with a microscopic examination is then best to look for microscopic evidence of blood in the urine.
Contamination. All laboratory results need to be interpreted in the clinical context and the doctor who ordered the tests is usually in the best position to do that. Talk to the doctor who ordered the tests. Having said that, if there are no other abnormalities, trace blood in the urine of a female is not usually a matter of concern.