Pro did my rough in. I'm installing outlets, switches, etc Using $20 no contact voltage tester - with plastic tip that beeps/lights when line is hot. I get beep/light even if line is absolutely, definitely dead?

For example: 12/3 wire direct from panel to switch box Nothing between panel and box. Red hot. Black hot. I put each on separate breakers, i.e. 2 separate branches. I flip red line on/Black off. Tester beeps for both red and black. In fact, black line feeds push pin hub with 3 pigtails to 3 light switches. All test hot. Hook up 3 lights. Red on / black off. RETEST. Red still beeps hot. Black now dead. And no lights. I flip black on. Yes, tester shows hot at black lead ends. Why the false positive hot when I know there's no current? Have tried separating black and red wires when testing. Is tester reading current from red which is on, even though black is off?

Some lines already rocked in. These false hots makes it difficult to diagnose electrician's rough in wiring at times.

Am I missing something?

Those testers pick up the field of electricity around the wires if the black and red are still incased in the cover or even if they are separate but close to each other it will light and buzz when one is hot.. if both breakers are off you should get no light.. Those testers are good for certain applications.. I use it to verify power is there for troubleshooting I never use it to verify a circuit is dead.. buy a tester with 2 leads that goes hot to ground and verify wires like that..

It's your tester, Inductively picked up. Don't wory about sheetrock being up. Just put in recepticles, plug in lamp. Will be OK. Easy and cheap tester is rubber bulb sockets with 2 leads, maybe get 2, tie the white of both to neutral, and black for one bulb, and red for other bulb, turn on each breaker, and verify operation.
Actually the test results did show both breakers were good, when either was turned on, it beeped/lit.

Mine is adjustable. Turn it up too high, and I get false positives. Fiddle around with it on known wires first.