Sierra Positive / Negative ?

[GeekNJ]

Having to work from home, I'm sure I'm not alone with the new ability to listen to the scanner in the background. I've heard this many times on the radio but I haven't been able to find it's meaning - Sierra Positive / Sierra Negative. It is usually said by a dispatcher to police or EMS. Any idea what this means?

Many thanks!

 

[a417]

if it started only recently , it might be sooper sekrit squirrel code for COVID-19 or something. Our state started using U21 pos / neg if the info is available.