You are referring to the EtG test. This test doesn't actually detect alcohol it detects ethyl glucuronide, or EtG, a byproduct of the metabolization of alcohol, which remains in the system for about 80 hours. There is indeed a problem with this test in that it cannot distinguish between consumed alcohol or alcohol in food or sanitary products. Different counties have different policies, but the way this is dealt with is that your numbers have to be above a certain level before you "fail" the test. Studies have shown that use of hand sanitizers repeatedly throughout the day for the 80 hour period can yield EtG levels as high as 1,500 nanograms. By contrast, a single drink (beer or shot) will yield you 6,000 nanograms. So you see it shouldn't be too difficult for the test analyzer to distinguish between actual alcohol consumption versus incidental alcohol intake. Low level results should either be ignored or flagged for caution but should not be used to revoke probation or driving privileges. I'd expect probation officers to instruct probationers to avoid using hand sanitizers or other high alcohol products/foods.
As for how this is admissible, well the State is held to a lower evidentiary standard for violations of probation as opposed to regular guilt/innocence trials. The judge hearing the evidence will take the results into account and grant them what weight he/she wishes. The science doesn't have to be perfect. As a practical matter, probation officers will almost never revoke probation just for testing positive. It would take multiple failures or other violations to justify a motion to revoke probation.