There is a compromise with ambushes. In situations where you can't take a perception test for seeing the ambush (someone attacking as you go around a corner say) you can still get a perception test to be alerted a second before the attack. It gives a +3 on the surprise test which could make the difference.

Also remember sometimes there is a difference between perceiving something and percieving something as a threat to you. For example there are tons of drones in the air over downtown. So just noticing a lonestar rotodrone near your area wouldn't neccesarily set off any alarms until it alters course toward you.

If your Perception is so high, you'd be able to hear or smell or thermographic vision or somehow overcome the penalty to detect the ambush.

For example, tinted/mirrored windows - Hidden ambushers, so Threshold 4 for the Perception check. Any other mods you care to add? Maybe it is on a moonless, starless, lightless night and somehow there's thermal smoke. So no matter how you look at it, it is -6 dice. Fine. Threshold 4 with -6 dice. So the dice pool needed is 4*4 + 6 = 22. Only 22, a Perception adept has usually more than that. Sure, as a GM you can say, "no, there is no way, you could have detected that ambush", but you could have just told the player that there's no way you are allowing his PC to detect your ambushes and he should just bugger off and make a "normal" PC like the rest of the guys, like John's gazillion dice melee monster over there.

Ok well now you're smacking into some of the fundamental issues with SR4's system, which "works" best around a range of dice pool sizes.

Still however it is imporant to remember that your character is not superman and therefore doesn't have X-ray vision (unitl the next sourcebook anyway). Similarly while techinically you could argue detecting a whisper in Bejing while you're in seatle would be threshold 4 with a -2 modifier for being far away you'd be a sap to let that happen (if you want to be technical write up a grenade like table for target modifiers for sounds based on range.)

However the trick is that you shouldn't be a dick about it. If they bring in a perception adept and suddenly every sniper is in a car with one way windows, and every ambush is somehow rendered indetectable you're being a dick. So usually they should get their chance at detecting stuff. They can hear the sound of multiple people breathing around the corner or what have you, let them have their fun.

But sometimes.......


*EDIT Also for things like a drone swinging it's weapons around remember you both can have your fun. The adept may notice it.

But it's going to try and shoot the troll first.

Here, I suppose, is where RAW surprise rules work for you. If the adept has high intiative nobody can hear their warning, and if they have low initiative they won't be able to shout it in time.