How do I know if the person read my message or Facebook messagner acting up?

I message someone on Facebook messagner and everytime I message them today it goes straight to seen like skip delivered in all. It doesnt make sense because how are they reading them so fast unless they have it open or literally looking at their inbox as I type but even then still have to click the inbox. It keep saying seen on every message and these messages are hours apart. This happen to me before too and I thought the person was ignoring me, but when I checked it again it went from seen to delievered the next day. It also looks like they not even in messanger because they dont have the blue symbole just the gray Facebook logo