I'm not too sure if loose items exist in Runescape.
If you lose certain items, even within the wilderness, you can reclaim them from Diango, Ianto, Azibo, or through your bank interface. I don't have a list to hand, but from memory the reclaim feature includes holiday event items and treasure hunter items. Both members and non-member items are included.
Edit* Here's a list: https://runescape.wiki/w/Diango/Reclaimables