I wonder if this is just something being lost in translation. The bars extend down to the shoreline, so saying "the bag/purse was found at the bar" could mean it was found on rocks directly outside, but still considered to be part of the bar's territory. Photos of Police Beach show the rocks. If the bag was by the bar, I can imagine someone saying "the bag was found at the bar" as opposed to at her hostel or in the jungle. Remember there's going to be a lot of second-hand knowledge getting to the press, and few Khmer speak perfect English. It's also possible that someone initially found it on the rocks and handed it in at the bar, in which case "the bag was found in the bar" and "the bag was found on the rocks" are true, in a way.
Having been to similar parties myself, it wouldn't be unusual for a group to go for a paddle or a drink/smoke somewhere a bit further from the music, put down a bag and forget it, then wander back without realising at first. Most people there are going to be drunk and high. I expect there are multiple bag/phone/wallet losses every time one of these parties happens.
Personally I think Amelia lost her bag, decided she'd had enough and wanted to go to bed, then came to some kind of harm.
Not that they should be viewed with suspicion, but I think it's pretty odd that no-one she was with on the night has spoken to the press yet. She's clearly pictured with people and they'd know what state she was in, if she'd lost her bag, who she'd been talking to, etc.
