Radio Reference is the best source, although it's not complete. You can find more by googling around, using the scanner to search yourself, and FOIAing various records.
Trunking is a way of allowing multiple people to share a set of channels without interfering with each other by separating users into Talkgroups rather than radio frequencies. A controller than which frequency pair a user on a talkgroup uses every single time they key up a radio.
Not sure which valleycom frequencies you inputted, but Kent PD is on the Seattle-King County Public Safety trunking system, so that's what you want to enter. If your scanner supports Motorola Type II trunking, you ought to be able to listen in. Just find the simulcast that is strongest in your area (probably the Kind County simulcast) and roll with that. To the best of my knowledge, RR is complete for the S-KCPS system.