Can subserosal uterine fibroids cause false positive home urine tests?

Fibroids. I presume you mean urine pregnancy test. The answer is absolutely no. Fibroids do not produce pregnancy hormone.
No. Uterine fibroids can cause false positive increases in certain other biomarkers, like ca-125 or he4, but they do not cause an increase in beta-hcg which is the biomarker used to detect pregnancy. A large subserosal pregnant may cause you to look pregnant, but the pregnancy test should be able to clarify.