Keep asking your wife :)
Really though, that is your best option. You will always overlook things outsiders to the game will notice, simply because you might have an X number of ways of how the story could develop in your mind. So your best option is to work with someone who is outside the game and whose imaginations on things like that will not be limited by system rules and tropes.
UPDATE: Also, don't forget, you don't have to write down every single detail. Have the general plot in mind and then if your players give you something you haven't planned on, make it fit then and there. You'll be surprised by how many things the players will think of that you hadn't, so don't try and cover them all, do it on the fly.
Good question. There's usually a deeper motivation behind going out and stealing things. Your characters will probably fit into one of the following three buckets and this will give them a reason to start.
Greed
A greedy character often wants to use money as a way to keep score. Thievery is an easy way to get it AND bring somebody else down a level.
This can be considered the default for most D&D style rogues.
Mastery
Somebody who is good at a skill will often want to improve it. They could have started out being trained by older mentors (small hands and an innocent look make for great pickpockets) or stole for survival. After more and more use, this then becomes the only skill that they think they can use.
At any moment, a chance to use and improve those skills becomes something that someone will try to take. The bigger a risk, the bigger the thrill when you succeed.
Survival
This is usually the way beginning rogues get involved in "the business". It's better than starving and there aren't any other chances. Once you've started stealing, it's easier and easier to justify doing it just one more time.
If anybody knows, blackmailing the character to do some more dirty work gets easier. Each time they break the law they get deeper and deeper. This is often how dirty cops in fiction get buried.
How To Use The Motivations
The motivations themselves can help you get players onto the edges of a big quest.
Telling a greedy character about a legendary dagger that is rumoured to be able to kill the gods is a sure fire way to get him looking for it. Make the clue-chain reasonable and he will beeline straight for it.
Tell a survival oriented character that he and his family will be well taken care of. They just need to do another little job first... oh, and another. Because we have your darling little kids.
A character that wants mastery will generally be intrigued by a challenge. NPCs complaining that the security in a particular building is too tight and they can't see a way in should be enough to perk his interest. Have him find something interesting that hints at a bigger challenge somewhere else and he'll be raring to go.
Beyond The Motivations
The most effective way to make players focus on a Big Bad is for it to threaten them specifically. Well, not exactly them, but their way of life and the world around them. There
During the initial phase, whether they end up working for the Mob or going freelance, encourage them to build up ties (and show off any backstory). These are what you are going to threaten.
Have one of the PCs mothers invite him for a visit occasionally. 2 minutes of "She offers you tea and asks after you and the boys you grew up with. What do you tell her?" should be enough to help set the relationship and make her exist in the player's minds.
They might also build up a relationship with a favourite fence, pawn shop owner or barman. They could take ownership of a building as a hiding-place, to cook drugs or run a brothel in. Whatever they feel like.
The purpose of this phase is to let them build up bits of the world that are important to them. It's important that they choose these, so you'll need quite a few pieces of colour that you can expand on as their interactions deepen.
Later on, you start to use the ties to build tension. For example, have the mother talk to the PC about strange men prowling around (and there's nobody there when he checks) a couple of times. Tell them they can't find their fence, that nobody's seen him all day and that his place was tossed.
Once they find out who their enemies are, you can bet your ass they are going to go out there and deal with them.
Summary
At the end of the day, to go beyond petty theft and feathering the nest, it's all about people. Who they like, who rubs them the wrong way, who seems to have a similar agenda, who can get them owing (and paying) favors.
Best Answer
Don't.
This is something I started to do in my campaigns. When the players derail the plot, I build a new one for them to follow. If they want to focus on how the bad guys have tech that isn't public knowledge, they can. They're ignoring the larger problem of "oh crap, zombies" while doing so, however, so simply let the rest of the world go on into decay as they mess around, until they're forced to confront the fact that the secrets being kept are perhaps a little less important than the fact that everything's collapsing around them. Let the plot equalize itself over time by letting the players do what they want; eventually they'll realize that they can expose this whole secret and conspiracy stuff, but don't have anyone left to expose it to because they've all become zombie food.
However, if you must.
Make them choose between their mission and the mission.
Eventually they have to be confronted with the following conundrum: Is our personal investigation getting in the way of the good of mankind/not being eaten by zombies? If they don't do this, then they'll continue their investigation as planned, but the world around them effectively ceases to exist as it was, meaning that they have to take responsibility for everyone dying and they no longer can get any help. If they're working with someone, have them pull support. Do so in a way that enhances the narrative, however "New York's almost overrun! Get back here or don't come back ever." works better than "and they didn't want you doing that.", because one sounds like a logical reason, and another sounds like railroading.
Dry up their leads.
If you really want them to stop searching for evidence, have the other corporation collapse. Their secrets and technology are gone with them as they crumble from within. The players could still try to investigate, but there aren't any real survivors to explain things, and anything they find essentially ends with "and now we're all gonna die".
Wave it away.
One thing it sounds like you're asking is how to explain the presence of high-tech gadgets. Simple; they're one-of-a-kind and untested. Sure they do what they're supposed to, but they're nowhere near finished. They're prototypes sent into the field. Ultimately, it's sort of your fault for introducing it if you can't explain it, but simply have it end. No more supertech gear because it didn't work, as the field trials showed. The only one got destroyed, beat up, or otherwise compromised and it proved to be less valuable than its investment.