Hello. Usually you can get your answer here
If you don't get an answer that doesn't mean that people are unfriendly, they just don't know.
While in battle, press F4 key to see detailed damage info and hit vectors. This way you can understand what's happened in most cases, i.e. why halftrack was destroyed by rifle fire or why this silly AT gun just knocked out your fancy tank.
Why can't I see the AT gun that is right in front of my tank after carefully inching up to it using the terrain for cover?
This depends on situation as there are many factors influencing that. After all, it's much harder to overlook a moving big tank than an AT gun behind some foliage. You can use LOS line to understand where you can see and where you can't (click on the, say, ground attack order button and survey the area looking at the line being shown from the selected unit to mouse cursor). Green is an area close to unit where it ignores foliage and can see through it, red is normal LOS and black means there your unit can't see.
45mm AT guns have some APCR ammo if i'm not mistaken, at close ranges this would be enough for a Sherman front plate. Use F4 key to clarify this.
Units can see through a foliage or smoke if they are close to its edge, while an enemy far away won't be able to see them. This makes ambushes possible (i.e. hiding an AT gun directly behind a bush). Becouse of this, lay a smokescreen between your and enemy units and not directly on top of enemy units or close (several meters) to them.
To summarize, yeah, stupid things can happen and make you want to smash something fragile and completely unrelated to the game itself, like monitor, but in general this is a fun game when you get into it.