As far as general investigative adventure creation, there are a few things that I've found helpful for d20 Modern:
1) Tell your players in advance what bonus they need to have to be major players. My rule of thumb is "14+level" for a retry-able or non-critical skill check. The Snoqualmie adventure was designed for 6th-level players, so there are a fair number of DC20 skill checks, and many DC30 skill checks for skills that allow taking 20 trivially. The players knew this going in, so there's one guy with a +11 Investigate check (and possible synergy bonuses in various situations), another guy with great Diplomacy, and another guy with Knowledge(Arcane) and Computer Use all over the place.
2) Have stuff written down beforehand -- when I didn't have it written down, I tended to just fudge and give out information as I felt appropriate. My players figured this out and told me that it felt as though their skill ranks were worthless, since they'd never find out more than I was ready to let them know. Now, I've got it all written down -- sometimes they get information a lot earlier than I'd thought they would, and other times they blow a roll and miss an obvious clue. It evens out in the end, and they're much happier.
3) Consider having different responses for different levels. This isn't spelled out explicitly in all cases in the d20M book, but it's already there for many skills, and it makes sense to have one bit of information for "Gather Information:15" and slightly better information for "Gather Information:20" and so on. When players realize that there are levels of success, they stop automatically taking 10 and sometimes shoot for that big lucky roll.
4) Write down Knowledge stuff beforehand. I'm lazy about this, but it makes the players who have Knowledge skills feel better. Otherwise, they say, "Do I know anything about that?", I roll for them and then infodump ("Why, yes, that specific arrowhead is commonly believed to be used by the ancient Anasazi Indians, and is definitely not Hopi or Navajo"), and then, in theory, their character gets to be cool by knowing that, but in reality, the player doesn't get to be cool by telling the group, since I, the GM, just told everyone. Writing that stuff down lets the player deliver the information in his character's voice. Not important all the time, but helpful in some cases, and it's a nice bone to throw to the guy with all the Knowledge ranks who doesn't feel appreciated from a "Getting to do cool stuff" perspective.
5) Roll some stuff in secret. I usually go for most mental checks (Charisma, Wisdom, Intelligence), so the PCs don't know how diplomatic they were, or whether they found no clues because they missed them, or because there were no clues to find. I break the rule from time to time, but at least in investigative and clue-searching modes, it's vital that the PCs not know what they rolled, sometimes.
6) Vary the circumstances. If the PCs can always take 20 to Search, then the random elements are lost -- you either make the Search DC less than 20+(their bonus to Search), and they always find it, or you make it a bit more, and they always need to get aided by friends to find it, or you make it a lot more, and they never find it. In any case, the ranks are essentially wasted, because there's almost no random element -- it's all GM-created. Adding in things like time limits that stop the players from taking 20, or red herrings that make it difficult to tell what's a clue and what isn't, or items that require Knowledge checks to identify as meaningful, make skills come in handy -- and they give the players a shot at success without a guarantee of success.
7) Be somewhat vague in your descriptions -- make them use their skills to get specifics. Remember how in D&D, you'd describe a trap, and players would try to disarm it without having ranks in Disable Device, saying, "Well, if I can just stick a stick into the little gears over there, that pretty much does it -- why do I need ranks for that?" If you describe blood spatters in detail, players who have watched CSI are going to say, "A-ha, slashing weapon, multiple attackers, based on the spray pattern," and then you'll either have to agree with them or say, "Er, actually, you don't think so..." because you, the GM, didn't intend that. I've found it helpful to just say, "There's blood spattered on the walls," and leave it at that until an Investigate check convinces me to let the PCs put more together ("A-ha, slashing weapon, multiple attackers, based on the spray pattern, and the wound placement indicates that at least one attacker was left-handed...").
8) Use lots of skills. d20 modern puts a lot of skills out there for players to use, and they can get a lot of ranks in a lot of skills, with a lot of bonuses as well. Beyond the obvious (Computer Use, Gather Info, Search, Research, Investigate, and Knowledge(Arcane) have been my party's favorites), I've gotten some use out of Spot (an item that would have been trivially easy to find with a Search, but which was well outside the range that people were searching in a grassy field) and Treat Injury (used to help determine cause of death and give the Investigator a bonus on his Investigate check). If your PC has ranks in Craft(Mechanical), let them use that to help them solve a mystery involving a murder victim who, for some reason, stopped her car by the side of the road and got out in the middle of the night ("A-ha, this hose has been replaced -- and judging by the fluid around the engine, the old one was leaking badly. My guess is that somebody cut the old hose, making her car sputter and die, and then replaced it after killing her so that we wouldn't know why she stopped. Easy to spot, if you know cars...").
I'm sure I'm missing stuff, and this is a bit of a hijack, but for people looking to create new adventures rather than use old ones, hopefully these might help a bit.
