I've noticed that is sometimes quite easy to go into neutral without pressing the clutch.
Does this cause more wear on the transmission? If so, how?
It depends when you do it and how quickly you do it.
If your engine was under a lot of load at the time, then the load would be briefly passed onto the synchronising clutches which could cause wear.
If the engine was not under load and you did it quickly, then it wouldn't be much different to doing the shift with the clutch pedal pressed.
With skill and lots of practice you can change gear without the clutch at all.
Move to neutral as you come off the accelerator (reducing the load on the gear teeth), match the engine speed for the next gear and slide it in.
Warning, you need a good "feel" otherwise earplugs as the sound of grating teeth...
Which means you don't have to damage the box at all.
Some of us learnt manual with gearboxes that did not have synchromesh on first or second gear so "matching" engine speed became second nature.
If you can shift it into neutral easily without force IMO it's pretty harmless. It will only be easy if the drive train is idling, that is, there is no power being transmitted either by acceleration or overrun.
It causes more shock and wear in the opposite case: engaging a gear from neutral without using the clutch while (obviously) moving. When engaging a gear, the rotational speed of the gear wheels on the engine side of the gearbox has to be matched to those on the output side, helped by the synchromesh system. When the clutch is disengaged, it's only the momentum of the gear wheels themselves that has to change. But when the clutch is engaged the engine itself has to change speed too, and it's much more massive. So that puts a huge shock on the gear wheels and bearings.
However, it is possible with practice, and should only be employed in the event of clutch failure. The technique is similar to the double-declutching system used in the days before synchromesh. It is essential to match the engine speed to the gearbox speed before engaging the next gear from neutral.
Suppose I want to change up a gear, the engine speed has to decrease. My technique is to first reduce the power until I can slip the gear into neutral, and then apply very gentle pressure to shift into the next gear, while letting the engine revs decrease. When the parts of the drive train match rotational speed, I can ease the gear in without force.
"Don't try this at home."