Here's a better question. Who lives in the noble district? I'll assume Forgotten Realms since you mentioned the Zhentarim.
My memory of the Forgotten Realms suggests that the noble district in a typical city is the province of:
1. Merchants. Such merchants would typically be members of a trading coster. They have reasonable amounts of cash and jewelery but the bulk of their wealth is invested in trade goods. The most valuable thing that a thief could actually steal is probably their records and correspondence, but those will probably be kept in a locked (possibly arcane locked--it's not very expensive and is fairly low level and thus reasonably available in the Realms) secret compartment somewhere. Anyone stealing such records and/or correspondence would need to find a buyer beyond simply a local fence--bandits or rival merchants would be the ones to pay for them. On the other hand, dealing with either is dangerous because both groups are unscrupulous (the rival merchant is, after all, purchasing trade secrets or business plans for nefarious purposes; scrupulous merchants probably wouldn't do that). And once the thief has handed over the documents, he is a liability (witness) rather than an asset.
2. Old money. There are fair numbers of old money type people in the realms. Some of these are useless fops living off the earnings of their ancestors. Others are heavily involved in commerce or politics. The useless fops are the most distinct from other classes because their extravagant lifestyle demands that they have significant assets in jewelry, expensive clothes, and cash on hand. On the other hand, the prodigal and his money are soon parted so the thief could easily discover that the only clothes and money the fop owns are on his person and the only thing in the impressive house is a stack of unpaid bills.
3. Leaders of various groups. The high priest of Helm, captain of the guard, and the head of the local thieves guild may well live in the noble quarter. It's at least my impression that, in the Forgotten Realms, those kind of positions often come with considerable personal power. Lord Morgrym of Shadowdale, for instance, is a fairly formiddable individual in his own right. These kinds of residences would be well designed (to minimize the possibilities of stealth) an well guarded--probably by dogs and guardsmen. And the personal capabilities of such individuals should not be overlooked. Even the safe of a mid level priest in the forgotten realms is probably protected by a glyph of warding.
4. Retired adventurers. They're all over the place in the Realms. Just becaue he's living in the noble's district and his 2000 gp is the +1 longbow hanging over the mantlepiece doesn't mean that your 7th level rogue wants to tangle with the retired 8th level ranger, his faithful dog (animal companion), and his wife.
5. Wizards. A lot of wizards live in mansions just like anyone else who is wealthy, but my take on the Realms is that wizards are probably the most paranoid of all. Who knows but that a wizard's mansion is not patrolled by bound devils (bearded devils are great for this kind of thing since they are weak enough to be bullied, lawful enough to keep their word, and dumb enough that they may not be able to figure out a loophole in the contract).
Then, as for the practicality of it
1. Don't assume that guards all suck. Your ordinary human 1st level warrior guard could easily have 2 ranks of spot or listen cross class, the alertness feat, and a 12 wisdom for a total of +5 to spot and listen and could easily carry a tanglefoot bag or bolas to try to take down an escaping thief (+1 or +2 vs touch AC is very doable). A higher class of guard (more competent but still quite affordable in the noble district) might be a 2nd level expert with 5 ranks in spot and listen, alertness, and a 12 wisdom. That's +8 to spot and listen.
Then consider the scenario. Assuming that a guard is in line of sight at all times but the rogue has cover and can attempt to hide, he'll probably need to climb the wall (hide check 1), sneak across the lawn (hide check 2), pick the lock (which will be a good lock at the minimum so several more rounds--hide checks 3-7), sneak into the building and steal stuff (hide checks 8-12 for a quick once over or assuming that he will be out of line of sight for most of the interior looting which may not be a good assumption), sneak out (hide check 13), cross the lawn (hide check 14), climb the wall (hide check 15) and hoof it. If the guard needs a 20 to spot the rogue on every one of those rounds, there's still a 59% chance that the rogue is spotted before he leaves. If the guard would spot the rogue on a 19 as well, it goes up to an 81% chance that the guard will spot the rogue before all is through. Add in that the guard also has a chance to hear the rogue (move silently) and, unless the rogue is 10+ points better than the guards and is taking ten or 19+ points better than the guards, the rogue will probably be noticed.
Then there is the possibility of revenge. People who can afford to live in the noble's district can hire good investigators. And ruthless people who live in the noble's district might take pleasure in making the rogue's demise as slow and painful as possible.