When I setup a VHF P25 site, I messed with Site Trunking.
I found with it selected that my G5 would start receiving an 800 MHz site which is part of the VHF site's system I had programmed.
Even though I never programmed the 800 sites frequencies, the G5 picked the info from the data stream and tuned to the 800 sites CC frequency.
The G5 display also indicated which RFSS and Site it was receiving from which is what told me what it had done. Had I not noticed, I'd have missed traffic as many users are not allowed on the 800 site.
When I unchecked Site Trunking, my G5 has always stayed on the site I programmed.