That frequency is part of a trunked system. Other traffic (like responses) may have been popping up on the other frequencies of the system.
Trunked System - Motorola Type 2
413.700, 415.150, 415.550, 415.950, 416.950, 418.350
Base 412.000, Spacing 25
TGs reported heard
8592
8752
8784
8848
16016
17040
17248
17296
17392
17776
18432
20816
21136