Assuming you are using Alternative 3, then check your Holding Procedures. Most establishments follow the "FSIS Compliance Guideline: Controlling Listeria monocytogenes in Post-lethality Exposed Ready-to-Eat Meat and Poultry Products" which recommends that on the first positive to take corrective actions then retest. If there is a second positive then you will need to hold and take samples of "products". From what you stated it looks like you have followed the correct steps. USDA might ask questions but wont' take any actions because it has been taken care of correctly. At most USDA will ask what you did and might want to observe the employee who samples the areas just to confirm. No need to notify USDA it is their job to request lab results as per their job duties, if they do see the positive then just have your corrective action paperwork ready.
Sources: FSIS Compliance Guideline: Controlling Listeria monocytogenes in Post-lethality Exposed Ready-to-Eat Meat and Poultry Products (Page 15)
"Establishments must verify that the corrective actions taken after an initial positive test for Lm or its indicator organisms on an FCS in the post-lethality processing treatment are effective. This is achieved by performing follow-up testing for Lm or an indicator organism after the FCS positive test that includes a targeted test of the specific site on the FCS that is the most likely source of contamination and additional tests in the surrounding FCS area."
"If follow-up testing yields a second positive result, hold and test products that may be contaminated using a sampling method and frequency that will provide a level of statistical confidence that will ensure that lots are not adulterated."
