Not offline available can also come from a cloud service (e.g. Dropbox), which tries to save space and does not put a complete file on your computer, but only a proxy which you have to make locally available with a right mouse click command.
Thanks for you inputs @Willi Adelberger, tried to replicate this by renaing the asset (indd) file so that will be missing in source indd and export to PDF but am not able to replicate this issue. also treid with below combinations.
Missing link from Loacal / server.
Modified link from local / server.
Link same image with 10 rectangles to check WIP is causing issue
checked with indd, png, jpg.
shared path disconnected and relinked.
changing the display performance to Fast Display, Typical, High Quality.
by changing the transperency blend space.
same sub template with multiple rectangles to see relink issue.