I used to work customer service (about 5-6 yrs ago) at UPS.... including the "Lost Package tracking" department.
I can tell you that there are several reasons a package may get lost, and you can sometimes tell what likely happened based off the last information you see when it's tracked. If it's showing as "Delivered", odds are it's a driver issue. Could've been delivered to the wrong place, or some other problem. sometimes the clue could be in the comments (Such as 'left at side door', or 'left with neighbor'.)
If however the last scan was anything in Transit or "out for delivery", It could be almost anything. A popular cause is simply that for some reason, the shipping label fell off. (Sometimes if it wasn't on tight or had a loose corner, it could snag and be ripped off by another package or the machinery). When this happens, UPS's standard procedure when they locate a package they can't figure out where it's supposed to go (no shipping label or anything written on the box), Is to open the package and look inside for a clue as to where it's supposed to go... .such as an invoice, or something with the address. [if you ship a package, it never hurts to also put the address on a post-it inside the package]. If that doesn't given them a clue, Then it gets sent to the big Lost and Found warehouse and a detailed inventory and description of the package contents are taken and put into a database.
When you open the investigation on the lost package, They'll ask about the contents of the package. The more detail you can give them, the more likely they'll locate the package based off that inventory taken when they open the package.
Unfortunately, one thing I learned is that all those "In Transit' scans you see, and even the "Out for Delivery" scan, don't always guarantee that they physically had your package. There are certain points in the system where the package will be physically scanned, but the other "scans" within the tracking data are more often than not scans of the container which the package was supposed to be placed in. For any number of reasons, sometimes it won't end up in that container but you'll still see it moving within the system.
One other thing I'll note..... often when a package is located, either thru someone finding something on or inside the box with your address... or via a match within that lost and found database and your package description.... They'll end up printing out a new shipping label to get it to you. This new label will have a new tracking number, so you might end up receiving the package even if you don't see anything via the tracking number.
(Oh! and another thing I'd recommend.... If you shipped the package via a UPS store, staples, etc, instead of hopping online and printing out your own label, I'd recommend contacting the location you shipped the package from. If you can prove the value, even if you didn't insure the package UPS will/did usually automatically include up to $100 in insurance on every package, but that claim process is handled thru the shipping account. There are also times when there may be a clue on the box (such a partial shipping label still attached) of where the package originated from, but nothing to indicate it's destination. In that case, they may return the package to the originating location assuming that they will be able to resend the package where it was supposed to go. In either case, it wouldn't hurt to inform them of the situation in case they can assist)