@Eriko I am not sure if it's a translation error on the manhua's part, but in the lightnovel, servants cannot read their master's thoughts. They just have enhanced hearing so they can more easily hear their master's commands. The protagonist just has a habit of mumbling to himself often, so they can hear that.
Forgot to mention, but chapter 104 in the lightnovel explains the servant's enhanced hearing a bit more. Here is a small excerpt from this chapter. (This is not a spoiler):
Come to think of it, I've asked about the matter long ago because they reported that even my smallest sounds could be heard clearly. At the time, Seo Dawon hypothesized that the servents were sensitive to every small sound from my lips because they were usually summoned by calling their name.
In other words, it's not that I'm particularly noisy but that the system is at fault. It was utterly unfair; I clenched my fist, appalled and mortified that I was unintentionally wreaking auditory havoc upon their ears.
▪️ Seo Dawon and Choi Kyung-sik discuss how to get past their limitation on movement by posessing a dummy. Choi Kyung-sik says a dummy would be insufficient, but it would be possible with a homunculus instead. ▪️ Moon Isaak breaks into the house because he thought Lee Kyung was ignoring his texts. ▪️ Jealous Seo Dawon a few more chapters ahead. Small excerpt from the light novel below; Seo Dawon steals Lee Kyung's cellphone from his hands and looks at his favorited posts. Seo Dawon: "You pressed like on Kim Sangyoon's most recent post, huh?" Lee Kyung: "N-No. That's... I just liked it out of habit..." Seo Dawon posts a hate comment on Kim Sangyoon's page under Lee Kyung's account. Comment is as follows: [Choi Lee-Kyung: Even when you try you look ugly lolol] Lee Kyung: "W-What did you do!" Seo Dawon: "I just commented my personal opinion." ▪️ Han Kiseok, Lee Kyung's first love, sends him a text out of the blue. Lee Kyung and Han Kiseok agreed to meet up for a coffee or whatever (The conversation just flowed in that direction). Seo Dawon gets even more jealous. ▪️ Seo Dawon is a bit self conscious and feels inferior to Han Kiseok because he is dead.
@Eriko Yes! It was noted by Seo Dawon that Lee Kyung (or Yikyung going by the manhua spelling) probably mumbles to himself because he must live a lonely life. The implication is he has no friends so he got used to being alone/mumbling to himself.
▪️Han Kiseok admits he was mean to Lee Kyung in the past and wants to make up. Han Kiseok: "So... in the past... I was quite mean to you. Even after graduating high school and becoming a User, I often thought of you. I wondered how you were doing. However, whenever I tried to contact you, my courage failed me. For some reason, I thought you'd ignore me. Weren't you also curious? About my life?" Lee Kyung: "Of course I was curious." Han Kiseok: "Really?" Lee Kyung: "Yeah. Because... I had feeling for you, back then." Han Kiseok: "Ah." Lee Kyung: "Don't pretend to be ignorant now. No matter - You weren't very interested in me... I know why you wanted to apologize... I just wanted to tell you that it's not necessary. Actually, if I still felt hurt... I wouldn't have come out to see you. I would have ignored your messages... I'm honestly okay now. I'm here because I feel alright." Han Kiseok: "I know it's too late to say this... but... I went to your house about a month after I became a User. I wanted to talk to you." Lee Kyung: "Re...Really?" Han Kiseok: "Back then, I waited but you never showed up. So, I gave up and came back home... " Lee Kyung: "I'm glad we didn't meet... Back then, I also agonized over whether or not I should contact you. You probably never intended this, but... If we met back then... hmm... I probably wouldn't have ever gotten over you. It's probably for the best that we only met and had a conversation now." ▪️Turns out Han Kiseok has a crush on Lee Kyung. (Lee Kyung doesn't realize this, but all the servants do.) ▪️Looks like Lee Kyung unintentionally rejected Han Kiseok without even knowing.