Not sure I understand your question completely, but they show up all along. After signing starts you can start looking for players considering only one DI human coached school and surmise that those players are fallback options for the coach if they lose battles elsewhere or perhaps if they just greened too many recruits at a certain position. You can then look at that DI school's scholarship situation, current battles, etc and decide whether it appears that coach will actually sign that green recruit. If you don't think he will then you can start spending to acquire him. If you can get him yellow and offer a schollie then when the DI coach fills his spots he'll sign with you, usually that cycle.
A few things to be aware of:
First, keep in mind other DI or DIAA schools are perhaps utilizing the same strategy so while patience is key you won't want to wait too long for fear someone else swoops in.
Second, if the player in question had SIMAI schools he was considering prior to being greened then he will sign with one those schools immediately unless you have put enough effort to yellow him
Third, if the DI coach never fills his scholarships for some reason the player may not sign with anyone
Fourth, the DI coach may end up needing said player and sign him after you've put some effort in
I've had success with acquiring these backup type players but there is competition for them and it's helpful to know if they were undecided or considering others in the first place.