You can successfully Investigate a location even if there are no clues on it.
- Success is determined in Step 6 of the Skill Test Timing, and committed cards get discarded in Step 8, so you cannot use Scavenging to get back an Item card that you've just committed to the same investigation attempt.
After you successfully investigate by 2 or more, exhaust Scavenging: Choose an Item card in your discard pile and add it to your hand.
Scavenging let's you repeatedly re-use "items" that are in your discard pile.
Some of most obvious cards in the current card pool that work well with Scavenging are items you discard to get benefits (like Knife), items you discarded because they ran out of "charges" (like Flashlight or .41 Derringer) or cards that simply tend to get discarded faster due to forced card abilities (like Baseball Bat.)
When you look closer, bringing items back into play with Scavenging doesn't come cheap. To get an item back from your discard pile into play you:
1) have to use one of your precious actions to investigate;
2) have to investigate succesfully by 2 or more;
3) have to use another action to play that item you just got back into you hand;
4) most likely have to pay recources to play the card.
Looking at these costs, it becomes clear that cards that combo nicely with Scavenging are cards that trigger another beneficial effect when investigating (succesfully.) A great example of such a card is Burglary. Investigate succesfully with Burglary while triggering Scavenging both gives you 3 resources and let's you take back a discarded item (which you can pay for with the resources you just gained.)
It's also beneficial if the items you bring back to your hand have no resource costs. In the current card pool cards like Leather Coat come to mind. With Scavenging Leather Coat becomes a resource-costs-free and re-usable damage sponge.
Cards like Sure Gamble increase your chanches of investigating succesfully by 2 or more, as does Wendy Adams' special ability. Speaking of Wendy, you can fuel her special ability by discarding an item card, which you can bring back later into play with Scavenging.
At the moment I don't think Scavenging is a "great" card. It's not a cornerstone that decks are built around, but examples like the Burglary combo show that it has good potential. In the end, Scavenging is as good as the item it lets you retrieve from your discard pile.
UPDATE 17-1-2017 After the release of The Dunwich Legacy, I took a look at some old reviews I did to see if the new cards had an impact on the reviewed cards. And man have I found a good one. In my initial review on Scavenging I mentioned the nice combo with Burglary That combo has even become better with new investigator Rex Murphy. Normally triggering Burglary's resource gain replaces gaining a clue when investiging succesfully. But when Rex Murphy uses it he still gains a clue due to his special ability.