It's actually on the top side of the board, right next to IC2 TK10420 pin 12-13.
It's a resistor that goes to ground when the squelch opens and will reduce the signal to the IC so it will think that the noise got lower and will keep the squelch open for longer when the signal goes weaker. If you remove that resistor completely there will be no hysteresis at all, that some scanners use, so you could try that first. Or that R152 value of 33k (333 marking for SMD or orange-orange-orange) can be replaced by 68k-100k to get a lower hysteresis, that actually are bit too high in the Pro2006. I might try this on my own Pro2006.
I did this kind of modification to a police scanner that had no hysteris at all and could sound like a stuttering machine gun at some times.
/Ubbe