Sparrow and Robin

[popcornscooby]

I'm not sure how to blast the cave door. Robin and Sparrow never turned up to the hotel again!

[SeaPhoenix]

After their first visit, Robin and Sparrow come back only if the library has spiders once again.

If you need a further hint: not hiring a certain employee for a couple days will cause the spiders to come back - I think a couple people allude to this during the game.