Assuming it's a Motorola ASTRO 25 system, you're seeing the behavior of a system at release 7.15 or older. The default BSI interval on one of the voice channels for ASTRO 25 systems prior to 7.16 was every 30 minutes. Keep in mind that 65536 is not an actual talkgroup, it's just a dummy ID that the author of Unitrunker gave to the BSI function many years ago. At no point does the system encode 65536 over the control channel to indicate BSI.I have seen TG 65536 used for FCC ID every 15 or 1/2 hour on another system