It is really strange that IAnnotation::GetAttachedEntities returns NULL in this case. This is reproducible for me as well. However this behavior may be used as the criterion to identify the offset dimension. I do not think any other dimension can have no attached entities. This is very risky because it looks like a BUG to me and it may be fixed.
Thanks for your input. I did bring up the issue of GetAttachedEntities3 returning Null with API support a couple of months ago. They didn't have a definitive answer to why this occurs but they didn't call it a bug. I requested an enhancement---for some way to get the attached entities or at least a way to get the offset value from the offset sketch relation. But I was still hoping someone might know of a good workaround for the time being.