
It's because you want to be polite and not just leave right after eating so that the host doesn't assume you're just there for the food
Since Demetrius never said "I'll excuse myself" yet got a response from Donovan like "You're excused Demetrius" tells me that Demetrius was ABOUT TO SAY IT yet Donovan said it immediately which means that he did in fact read his mind, notice in contrast Melinda HAS to say it mostly likely because her mind is preoccupied with being scared of Donovan, being a coward and waiting to protect her children.
Idk, maybe my tinfoil melted into my head but that's my two cents

Remember when Anya was unable to read Demetrius's thoughts, maybe because he is used to controlling his thoughts to prevent his dad from reading his mind, and thats why he doesn't involve himself much with other people.
And during the Dinner Demetrius intentionally thought "I'll excuse myself" to check how his father Donovan will react, and his father replied him by mistake.

So I'm reading it again too &
Chapter - 38 when Loid meets Donovon Desmond at school
He (loid) clearly states in his mind that he needs to finds everything about he's(Desmond) doing & the war he is plotting
So if he can read minds he must be aware of him & everything & i don't think there's any indication of it
So I Guess GETO's body taken over by Kenjaku left us with PTSD lol
But I'm willing to bet that he's involved with Anya somehow, either he facilitates experiment or he killed Anya mommy IDK
Ok went back and reread but did anyone notice in chap 106 at no point did demetrius actually say "I'd like to excuse myself" like I'm not the pinnacle of elegance but isn't more rude to just stand up and wait than idk excusing yourself?????