A couple of plot seeds which center around green boxes.

1) A skinhead gets caught either tring to rob a rare book store or is caught up in a raid and agents find a thing that should not be in the flop house of any skinhead. Items that are found to be filled in. A cell smells an old rat and sends in a cell to see what they can trun up, and finds some old friends.

2) Servicing a Green Box may be a task assigned to a recruit (or a new Cell) as a form of vetting exercise. Essentially - go here, do that, move this here, drop the keys in an envelope and mail it. All the while, the Agent is under surveillance to determine tradecraft level, reliability and ingenuity.

3) Do the above, but this time it's when the Cell is in another location for a NatO. Essentially - while you're in town, rent five different mailboxes and two storage lockers and forward the keys. Move the contents of one locker to another location.

Then, when you want a low Mythos session, get the Cell to recover the contents of a Green Box that has been robbed.