issue moving an activity from a project to another one

Hi !
I had to move an activity from a project to another one. This works fine, the activity no more appear in the initial project ans appear in the new one. Real work is fine too, the work is in the new project.
I tried to remove old project and Projeqtor said "No, you still have work into the project" (old one). But project have no activity, no meeting ..)
I guess the matter is in the work record. each work record reference both project and activity, and I guess when project linked with the activity changes, work records are not impacted. So they still reference the old project.

And I have a question : when I import work records, coming from another tool, I only reference the activity, not the project. Thre column is not present in the import file. This works fine, but is there a hidden impact ? Is project id mandatory in work records ? 
And more : how to know which fields are mandatory in an import ?

Thank you so much for your help !

Time is declared per activity not per project. Project "inherit" time from all objects inside. So if you moved actviity to another project you moved also workload.

What you can do is to open planing view and display your old project. Check value of field "Left" for project. You can close it only if Left is 0.