Originally Posted by
MTCman379
I am almost positive that any Delta flight will operate with transponder on. Sometimes there are 2 day aircraft maintenance checks, and sometimes there are unscheduled maintenance event troubleshooting where maintenance estimates a ready time, which then gets the aircraft added back to the schedule with a buffer, but maintenance takes a little longer than originally planned.
The transponder will be on for ATC purposes but airlines/operators can request that flights be blocked from flight-tracking apps for privacy and security reasons. I'm not suggesting this was the reason for sure and the MX theory (either scheduled maintenance taking longer than expected or unscheduled maintenance popping up) is the more plausible candidate, but I was simply offering another possibility (that the aircraft was operating some sort of charter that was blocked from any flight tracking app/site).