Not sure what's going on in Loki's head there, but one possibility is that she's wondering if her grandfather was a trickster in disguise. Though exactly what motivation a trickster would have for deluding someone into being happy is anyone's guess. And even if walking through the door was some kind of illusion, they'd still need to have got inside.
I don't think Loki has any reason to doubt Tella being a ghost. Through volume 35 she seems pretty certain still that it was Tella and in Volume 30 she's the first one to piece together that Mana is a ghost. If anything, I think she's putting pieces back together again. The 'she can appear any age' and 'her eyes are the same as grandpa's' is really her only back up on Mana but I'd love to see Darius call Tella and trickster. That would probably piss Loki off big time.
Quote from: Puyon on January 24, 2016, 09:42:26 AM
If anything, I think she's putting pieces back together again.
Oh, of course - I'd forgotten that Drake made her forget.
EDIT: Be that as it may, I can see Darius not believing in ghosts and assuming that Tella was a trickster too.