Yeah, if a friend of mine's sister went missing and he then asked me to hide bags/boxes of stuff in my shed I guarantee I'd be checking what's in them, I don't think any of them are that naive.
I'd guess that this all happened before news of Becky's disappearance hit the headlines, probably the night before she was reported missing. So it's not out of the question they either didn't check what was in the bags until they heard about Becky disappearing, or at least that they didn't realize who the body parts belonged to - could've been a "don't tell me anything mate, I don't want to know" kind of scenario...