My milksnake likes to find a way behind the foam rock wall and hang out against the glass. I try to move her and set the wall more securely, but she always finds a way back there again. Anything to worry about? Is my tank too warm for her? Or does she just find this area cozy / safe. The terrarium is 3' x 2', and she's grown to about 5 ft.
Usually when this happens, it is because the snake feels there is a lack of adequate hiding enclosures in the cage. As you can tell, a snake loves to squeeze into the tightest space possible that can still cover them. If it chooses a hiding place behind the wall, it is probably doing so because of a lack of other hideouts, the other hideouts are too big and don't help it feel secure, the hideouts are too small and it feels exposed, or it doesn't like the hide/material of the hideout or it prefers the wall. I would say, maybe pick up another hideout or two (two being the preferred if you don't have any) from a pet store or online, then try to reset the wall and see if it still prefers that.
If that doesn't work then I would say it isn't that big of a deal. If you really didn't want it behind there, you could try to hot-glue the formation to the side of the cage but do not try to tape it. If there is something sticky in your cage, a snake will find out how to get it stuck on them (trust me), and hot glue isn't sticky or harmful when removed.
You can find an example of a hideout here.
A general rule for snake enclosure size is that you take the perimeter (length + length + width + width) and if that is less than double the snake length, you need to upgrade. in your case you have a 3 x 2, so 2*3 + 2*2 = 6 + 4 = 10ft (around 3 meters) which is exactly twice your snake's length. This means the size is fine, but it wouldn't necessarily be bad to upgrade to slightly larger one.
As for warmth, you should provide your temperatures of the cage or else we won't be able to tell you if it is because they are cold. Same with humidity. You can read more about the care of your snake by looking up a milk snake care sheet online.