My elderly mother had a phone conversation with me and imagined there was a lot more to the conversation than there was. Is this a behavior of dementia?

This is a red flag for you (or her MPoA) to get her in for a cognitive and memory test. If she doesn't have a DPoA, this should be created first preferably. She will need a legal representative to manage her care. Without this, helping her and making decisions/taking actions in her best interests may become very difficult, especially if she becomes delusional, paranoid, resistant, etc., which many elder with dementia do.

Have you been to her home lately to see what's going on in there? Is it dirtier or disorganized more than normal? Is there unopened mail? Or bottles of opened pills lying around? These are very common signs of a problem.

Another reason to get her in to the doctor is to make sure she isn't having a UTI, which, in the elderly, often presents symptoms that mimic dementia, like confusion, delusions, hallucinations, agitation, etc. This is testable and treatable. If left untreated can become septic and dangerous. Best to discount all other medical problems before coming to the conclusion that she has the beginnings of dementia and memory impairment. If you have siblings, it may be time for a discussion on how best to help her.

If it were my mom, the overnight visit plans would be more of what she wanted rather than a dementia type issue.

yes it could be.. when was the last time she has her coratid arteries checked? Is there enough oxygen blood flow to the brain? It’s a simple test not very invasive it at all..