Scavenging (for lack of a better name) is one of the strategic/tactical attitudes available to a player in EU4. Summed up simply it is the strategy of continuous surveying of the environment to find countries that are weak enough to be an easy prey. Sometimes it is a country that is dead, it just doesn't know it yet. Sometimes it is an overextended and weak country that has provinces it cannot defend or keep. Sometimes it is a country in a war or that has just been ravaged by war, that is now vulnerable to attack.
Normally a player has an overall strategy and needs to fulfil various steps to accomplish this strategy. Scavenging can be an important part of that strategy. If the player plans on expanding in an area and a scavenging opportunity appears, the player needs to be ready to grab it. At other times options will present themselves: all countries have moments of weakness. In general the weaker a country, the more useful scavenging will be. The big and strong countries can usually take what they want, a scavenger will patiently wait for the moment when the risk is minimal and the reward is big. A country with a strong navy also has an advantage because it can move fast, blockade and has an easier time controlling scattered provinces.
The two most useful game tools for a scavenger are the ledger and the diplomatic screen. The ledger in particular is useful as it shows the strength of armies, navies, economy and wars of a certain country. The diplomatic screen will show if a country has allies, if it is a vassal or in a personal union. Checking the HRE map is also essential, to know if there is an "invisible" "ally" ready to defend the "prey". It is also important to keep an eye on messages about new alliances and broken alliances. These will inform if there is a small window where a country has weak allies or even no allies. Messages about wars starting or finishing will also show when a country is vulnerable. In general the sum of the information will tell the player when a country has turned into prey. The smaller the nation, the more benefit from choosing weaker countries.
Risk against reward
As part of the risk-calculation the player should also factor in allies are and whether they will help. Good, strong allies that will support expansive wars are invaluable (e.g. France, Bohemia), where allies that primarily support defensive wars (e.g. Austria) are of less use. Naturally a scavenger will need to calculate if the reward is really a reward. A big strong nation will typically have enough forces to put into a rebellious new province, a "weak" scavenger needs to factor in that cost before starting the war.
A good scavenger will win half the battle before it even starts. By having an overall strategy the player may discover countries in the current expansion zone that are possible later preys. When the window of weakness appears the scavenger must act fast, so diplomats need to make a claim for the situation in advance. Relying on missions and their claims/CB is not reliable as the missions may be gone when the scavenger needs them. A Trade Embargo CB might also disappear if a country feels weak and removes the embargo. Sitting around for a year making a claim is a sure way to lose an opportunity.
While not as critical, it is usually a good idea to have a diplomat physically near so he can get back to the capital in a few days so war can be declared before a weak country gets new allies.
Absolutely essential is to have a reactionary force led by a general ready at all times, ideally with a navy to carry it to the location quickly. The strategy will of course vary depending on the prey, but in general sieges bring the most warscore. Getting rid of enemy forces is essential, and carpeting the country with sieges will prevent reinforcement. Using blockades, cannons and generals with siege bonuses will cut months of siege-time and a good scavenger will get the job done as quick as possible. Assaults will cut time, but in general a reactionary force will rarely have manpower to afford this.
In general, scavenging can be used by everyone, but the weaker the nation are the more it will benefit from this line of thinking. Patiently waiting, watching and building relevant alliances will typically beat premature aggression. All players should keep track of the ledger and the alliances and wars, but it is necessary for the scavenger. Play to the nation's strengths and the prey's weakness. If the scavenger has a strong navy, factor it in when finding relevant prey. With a strong economy using short term mercenaries should overwhelm many countries. In the case of better land forces, use them.