Since you're not the one planning the heist, the players are, you need to be more aware of where their ideas may/may not fail or suceed. A few things to keep in mind:
- people tend to become complacent about security when it's not drilled into them and/or there hasn't been a recent security event. This means things like not locking the doors because it's inconvenient, forgetting to set the alarm system, failing to replace necessary sensors, and actually overriding security protocols that have become a hassle, etc, etc.
So when the PCs are staking out specific ships, you should have some idea of how recently said ship/crew/security zone has had recent security concerns and have the personnel involved react accordingly - you can give them lots of subtle signs that a given ship will be harder or easier, and then let them make the choice. The slovenly crew that walks away from their ship with the cargo bay door left open frequently is likelier to be an easier target than the spiffy looking crew uniformed crew that is constantly using security access badges and closing doors immediately.
- most people don't question those who look and act like they belong there, unless it's an ultra secure environment or an attentive person, or a really small/closed environment. That means in a large facility where many people are - like a starport - people who act like they belong in a certain area are less likely to be questioned about their presence there. Of course, certain Hi tech tools like GPS tracking badges, etc, can negate this, but even in those cases, many people assume that the technology is handling the security - IE, "Well, if he's walking around here, and security hasn't shown up yet, I guess he has the right to be here."
Little anecdote - this was drilled home to me when two guys walked into a pizza parlor my company had video games placed in, told the guys behind the counter that they were swapping one game out with another game that they'd bring by that evening, and walked out with the video game - even getting an employee at the pizza parlor to sign a 'receipt' for the game they were stealing.
My point is, that in game, if your PCs act bold and decisive about being where they don't belong, they may actually have _less_ of a chance of being caught than if they are slinking around in the shadows.
- the more complicated the plan, the more likely things can go wrong. But even when things go wrong, that doesn't mean the plan fails.
- a lot of criminals don't get caught, or at least not right away. So what if the authorities figure out the starship they stole off the landing pad wasn't theirs before they make it to the 100d limit. Every bit of distance they get between the crime scene and themselves ups their odds of staying free - even if they think everyone knows who they are.
- sometimes the law can work against the authorities just as much as it can the criminals. The SDB boat following them through the system may know they stole the ship - but that doesn't necessarily mean they can just shoot them out of the sky at will, particularly if that shooting might put innocents in danger. Take a look at the policies of some cities regarding high speed chases for some examples of how the law can let criminals get away.
- Finally, stealing from the wrong person can be worse than the authorities catching you. A ship may be "unguarded" because everyone knows it belongs to Louie the Kingpin and that he'll crucify anyone who dares steal from him. Even the cops won't step in.
BTW, most of my examples were assuming they would be stealing a ship from a starport, rather than hijacking. A lot of it applies to hi-jacking as well, but you did say "stealing a ship". In any case, a lot of others here posted inof on hi-jackign, so I felt it was covered.
If this were in my game, how hard/easy it would be for the PCs to steal or hi-jack a ship would really be based on how clever the players were and the resulting actions of the PCs. Basically, whether or not their plan suceeds would really come down to what situation was more fun overall - a really exciting plan that required lots of skill rolls and resulting in lots of narrow escapes for them to successfully pull the heist off would win out over them trying to use brute force.
Especially if they're the ones who shot up your other ship. Then you can paint the ship black, paint some white skull and cross bones on the sides, and say "Arr" a lot while you plunder the Spanish - er, I mean "Spinward" Main for loot and rum.
