Leuc is short for leucocytes - these are the white cells that fight infection. +++ means quite alot were picked up in your urine. This could be due to urine infection or contamination with discharge (which is naturally high in white cells). It is usually unrelated to strep B infection which would be found on a swab.
Did you have any symptoms of urine infection eg. frequency (even more than normal for pregnancy), urgency, leaking, nausea, feverish?
Do you know of the midwife sent off a sample to the lab for culture?
Unfortunately its not routine to test for Strep B. You can get a private test for about £30 and, like StiffyByng (a PG Wodehouse fan by any chance?) I think it is worth it if you are worried considering the implications of a positive test.
I think your midwife would normally ask for another sample or send one to the lab to get double checked if you have +++ of leuc or protein in your urine. Especially late on in pregnancy, urine test results are often contaminated with increased vaginal discharge and give back results that look worrying but aren't. DO ask your midwife though - she definitely should have told you about your urine result even if there is no cause for concern to keep you informed about your health.