It was not a booby trap! He was entering his apartment, he wasn’t approaching the scooter. It was just there!

A soldier hiding in a bush is a combatant. The grenade in his hand is a weapon. It is easily discernible as such both by other soldiers and by civilians. And both soldiers and civilians will expect a grenade to explode when it is thrown at them, or at least they understand the risk of a grenade potentially exploding, if it is laying around. So they expect it to be explosive.

Yeah, sure, they expect it if they see it and know what it is. The grenade does not suddenly become a booby trap just because you throw it at someone that isn't looking, so the target knowing about it clearly cannot be a requirement here. Again, concealment alone is not perfidy. Inviting and betraying trust is. If you disagree, then I ask you again, quote it. Kirillov had no reason to give the scooter a second thought whatsoever; there is no invitation of trust from a scooter being parked at the side of the road.

The entire bit about definitions here hinges on the word "when". To me, "...which functions unexpectedly when a person disturbs..." means that the function is dependent on the disturbance or approach. To you, it just means at the same time. So let's look at the word "when", since you decided to quote the dictionary

If you scroll down to the grammar notes in your link, you will see "If or when?", a section about the usage of "when" as a conditional. So clearly we can't just ask the dictionary here.

You know what's much more helpful? The part immediately after that definition of booby traps.

5. "Other devices" means manually-emplaced munitions and devices including improvised explosive devices designed to kill, injure or damage and which are actuated manually, by remote control or automatically after a lapse of time.

Since manually-emplaced and -triggered munitions are "other devices", they clearly aren't booby traps under this protocol.

"Other devices" are still relevant to some of what you raised, particularly article 7.2 in this context. Whether a vehicle like a scooter counts as "portable" or not is ambiguous, as yes you can move them, but under their own power. You certainly can't (easily) pick them up and carry them. Since you've already referenced Lieber extensively, https://lieber.westpoint.edu/booby-traps-ukraine-conflict/ provides:

Second, booby-traps may not take the “form of an apparently harmless portable object which is specifically designed and constructed to contain explosive material and to detonate when it is disturbed or approached” (art. 7(2)). The U.S. Department of Defense’s Law of War Manual provides the example of “booby-traps manufactured to resemble items, such as watches, personal audio players, cameras, toys, and the like.” It observes that the “prohibition is intended to prevent the production of large quantities of dangerous objects that can be scattered around and are likely to be attractive to civilians, especially children” (§ 6.12.4.8).

In that regard, note that the provision does not bar the booby-trapping of actual harmless objects, such as cameras or houses. The prohibition only applies when the booby-trap is intentionally designed to look harmless, as in a booby-trap made to look like a camera. Nor does it prohibit booby-trapping non-portable harmless things, like a gate. And it only applies to booby-traps designed or otherwise manufactured to serve as a booby-trap. Accordingly, it does not apply to field-expedient booby-traps or otherwise improvised ones. More information on the nature of the Russian booby-traps is required to determine whether this particular prohibition has been violated.

In this case it's discussing Russian usage of such devices, so there shouldn't be a bias towards leniency. So by the sources you've been relying on:

• This wasn't a booby trap
• Even if it was, it still wouldn't be a war crime or perfidy