by the way, note: this is another proof of how uneasy it is for a person to understand what another person may not understand especially when we communicate in text. I suggest we all describe reasons for character actions to reduce confusion. I'll do mine:
- hellhound (h) was obviously looking for more spiders and he found them using his keen smell, while none of PCs were able to
- the invisible creature just moved through the wall to avoid Thorem attacking it, how it did that - you don't know as you do see it, only hear it
- spiders did not intend to attack medium sized creatures until you started poking at them (under the cloth)
P.S. I totally forgot that it not uncommon for DnD community to use custom rules for invisible creatures in a battle encounter, assuming that it is impossible for other creatures to know the location of an invisible creature. DnD rules make invisibility less powerful than that. I hope we are now clear about those rules as I may be putting more encounters where you may have issues seeing your enemies - be it invisibility, darkness type spells, fog or other reasons to have limited visibility or enemies at range >60 feet in non magical darkness.